Allah, the Wise, has said:
فَلاَ تَقُل لَهُما اُفٍّ وَ لاَ تَنْهَرهُما
"Say not to them a word of contempt, nor repel them" 
The Holy Prophet (s.a.w.) said:
بِرُّ الْواَلِدَينِ اَفضَلُ مِنَ الصَّلاةِ وَ الصَّومِ وَ الْحَجِّ وَ الْعُمْرَةِ وَ الْجِهاَدِ فِی سَبِيلِ اللهِ
"Kindness towards parents is better than prayers, fasts, Hajj, U’mrah and jihad in the path of Allah." 
In (several places in) the Qur'an, God has spoken about the very subtle issue of ‘kindness towards the parents’, attaching such immense importance to it that He has even said: Say not to them (so much as) "Ugh" nor chide them, and speak to them a generous word.
It is evident from this that it is not only forbidden to annoy parents in any way but it is obligatory to exhibit goodness and kindness towards them. People who hurt their parents, even occasionally, must seek their forgiveness and happiness before they suffer the evil consequences. They should remember that their children, in turn, would ill-treat them too.
As for the repercussions in the Hereafter, the holy Prophet has said that if for every instance of hurting others one door of Hell opens up for man, then two doors of Hell will open up for he who causes inconvenience to his parents.
1 – The Mother’s Pleasure
The Holy Prophet (s.a.w.) approached a youth who was dying and said to him: "O’ Youth! Say ‘Laa ilaaha illallah",but the youth’s tongue would not move and he was unable to speak the sentence.
The Holy Prophet (s.a.w.)asked the gathering crowd if the youth’s mother was present. A lady, who stood near the head of the dying person, stepped forward and said she was his mother.
The Holy Prophet (s.a.w.) asked her if she was pleased with her son to which she responded: "No. I have not spoken to him for the past six years."
He said, "O’ Lady! Forgive him and be pleased with him."
The lady agreed and said, "For your sake, I forgive him. May God be pleased with him!"
The Holy Prophet (s.a.w.) again turned to the youth and asked him to testify to the Unity of God and this time, having procured his mother’s pleasure, the youth was able to do so.
The Holy Prophet (s.a.w.) asked him, "O’ Youth! What do you witness at this moment?"
He said, "I see an extremely ugly person who has a foul odour coming from him and who is waiting to strangle me."
The Prophet (s.a.w.) then instructed the youth to recite the following supplication: O’ He, Who accepts trivial (good) deeds and disregards the immense sins! Accept from me my trivial (good) deeds and disregard my immense sins, for you are the All-Forgiving and the All-Merciful.
When the youth had recited this supplication, the Holy Prophet (peace be upon him and his holy progeny) asked him once more what he was witnessing.
He replied, "I now see a man with a luminous and pleasing appearance. He is wearing beautiful clothes and possesses an enchanting fragrance. He is exhibiting kindness andcourtesy towards me".(Having said this, the youth passed away).
2 – The Companion of Prophet Musa (a.s.) in Paradise
Once, while conversing with God, Prophet Musa (a.s.) requested:
"O’ Lord! I desire to meet the person, who is to be my companion in Paradise."
Jibraeel descended and informed him that his companion was to be a butcher who lived in a certain place. Prophet Musa (a.s.) set out in search of him and arrived at his shop, when he noticed a youth, resembling a night watchman, busy selling meat.
When night fell, the youth took some meat and proceeded towards his house. Musa (a.s.) followed him till they reached there.
Musa (a.s.) approached the youth and said, "Would you not like to have a guest?" The youth willingly agreed and took him inside.
Musa (a.s.) watched the youth preparing some food. When he had finished, he brought down a large basket from the upper storey. Bringing out an old and wizened woman from inside it, he washed her and then proceeded to feed her with his own hands. When the youth was about to carry the basket back to its original place, Musa (a.s.) noticed the old lady’s lips move as she mumbled something incomprehensible. The youth then brought food for him and both of them ate their dinner.
Musa inquired, "What is your relation with this old lady?"
The youth replied, "She is my mother and since my financial state does not allow me to purchase a slave-girl for her, I myself strive to serve and look after her."
Musa questioned further, "What did your mother mumble before you took her upstairs?"
He responded, "Whenever I wash her and feed her, she prays: May God forgive you and place you in the company and in the rank of Prophet Musa (a.s.) in Paradise."
Hearing this, Musa (a.s.) said, "O’ Youth! I give you glad tidings; God has accepted your mother’s prayers and Jibraeel has informed me that you shall be my companion in Paradise!"
3 – The Curse of a Mother
In the tribe of Bani Israel, there lived a pious person by the name of Jareeh, who used to engage himself in God’s worship, in his monastery. One day, his mother approached him while he was busy in prayer and, as a result, he did not respond to her. She approached him for a second time, but again, Jareeh did not answer.
When this happened for the third time, she became angry and cursed him saying, "I pray to God that He does not help you!"
The next day a prostitute came up to his monastery and giving birth to a child there, declared:
"This is Jareeh’s child whom I have given birth to."
This created uproar among the people, who thought: "The very person, who used to rebuke us for fornication, has committed it himself." The king ordered him to be sent to the gallows.
When Jareeh’s mother came to know of this, she began beating her face in distress whereupon he said to her, "Keep quiet, for it is due to your curse that I find myself in this predicament."
The people asked him, "O’ Jareeh! How can we know that you speak the truth?"
He told them to bring the child to him.
When the child was brought, he prayed and then questioned the child, "Who is your father?" The child, with divine power and permission, said, "My father is such and such a shepherd belonging to such and such a tribe."
This incident saved Jareeh’s life after which he vowed never to separate from his mother and to serve her throughout her life.
4 – The Truthful Barber
The distinguished scholar, Sheikh Baqir Kadhimi, who lived near Najaf-e-Ashraf, narrates that a truthful barber, once related the following story:
I had an old father, whom I served diligently. I exercised such great care never to be neglectful towards him, that I would even place water for him in the toilet and remain in wait outside till he emerged. All throughout (the week) I would be watchful of him, except on Wednesday evenings when I would proceed to Masjid-e-Sahlah hoping to meet Imam Mahdi (a.s.).
One Wednesday I was very busy and did not find a spare moment until it was almost sunset. Nevertheless, I set off towards Masjid-e-Sahlah alone in the dark.
It was a moonlit night and I still had one third of the journey to cover when suddenly I observed an Arab, seated on a horse, coming towards me. I said to myself, "This Arab is surely going to loot me", but when he had come closer, he spoke in the local dialect, and sought to know where I was heading for.
I told him that I intended to go to Masjid-e-Sahlah, whereupon he inquired if I had anything with me to eat. When I replied that I did not, he spoke with some sternness, and said:
"You have some food in your pocket".
Putting my hand into my pocket, I found some raisins, which I had purchased for my son but had forgotten to give him.
The Arab then said: "I recommend you to serve your father", and, repeating this three times, he suddenly disappeared.
It was only later that I realized that it was Imam Mahdi (a.s.) himself that I had seen and that he was not pleased that I stopped serving my father, even for the purpose of going to Masjid-e-Sahlah on Wednesday evenings.
5 – Striking the Father
Abu Quhafah, the father of Abu Bakr, was one of the enemies of the Holy Prophet (s.a.w.). Once, he abused the Prophet (s.a.w.) and so, his son Abu Bakr took hold of him and pounded him against the door.
When the news of this incident reached the ears of the Holy Prophet (s.a.w), he called for Abu Bakr and asked:
"Did you do such a thing to your father?"
Abu Bakr replied in the affirmative.
The Holy Prophet (s.a.w.) then said, "Go away, but from now on, do not behave in this manner with your father." 
Holy Qur'an, ch. Al-Israa (17), vs. 23.
Jaame’ al-Sa’adaat, vol. 2, pg. 264.
 Ihyaa al-Quloob, pg. 129.
Darshai Az Zindagi-e-Payaambar, pg. 116; Al-Amaali (Sheikh Al-Tusi), vol. 1, pg. 63.
Pand-e-Taareekh, vol. 1, pg. 68; Tuhfa-e-Shaahi (Faadhil Kaashifi).
 Namunah-e-Ma’arif, vol. 2, pg. 548; Hayaat Al-Quloob, vol. 1, pg. 482.
 Muntahal Aa’maal, vol. 2, pg. 476; Najm al-Thaaqib.
Daastaan-ha Wa Pand-ha, vol. 10, pg. 128; Wasaail al-Shia'h, vol. 1, pg. 115 (Old Publication)