Allah, the Wise, has said:
أَوْفُوا بِعَهدِ اللهِ إِذاَ عَاهَدْتُمْ
"And fulfil the covenant of Allah when you have made a covenant." 
The Noble Prophet (s.a.w) said:
لاَ دِينَ لِمَنْ لاَ عَهِدَ لَهُ.
"One, who does not adhere to his covenants (and promises), has no religion." 
In the Noble Qur`an, Allah has made many promises and has ordered that covenants and promises should be fulfilled.
One who enters into a covenant must adhere to it and not violate it irrespective of whether the covenant is with Allah and the Prophet, or with the creations of Allah. Non-adherence to one's covenant and promise shall cause one to become ostracized, its burden shall remain upon his neck like a necklace till the Day of Judgment and Allah shall cause the person's enemies to gain ascendancy over him.
Even if the opposite party happens to be a disbeliever or an immoral person, one must not break one's covenant with him and cause his life to fall into disarray.
1 – The Noble Prophet and Abu Haitham
The Noble Prophet (s.a.w) had promised one of his companions, Abu Haitham Ibn Tayyahan, that he (s.a.w) would present him with a slave. Incidentally it so happened that three captives were brought before him - two of whom he (s.a.w) gifted away while one remained with him.
Meanwhile Hadhrat Zahra (s.a) approached him (s.a.w) and requested: "O' Prophet of Allah! Can you grant me a slave or an assistant? Do you not see the effects of the hand-mill upon my hands?"
At that moment the Noble Prophet (s.a.w) suddenly recollected his promise to Abu Haitham and said to himself: "Since I had previously promised Abu Haitham, how can I grant my daughter precedence over him, even though my daughter turns the hand-mill with her weak and delicate hands?"
2 – The Conquered King of the Sasanid Dynasty
The Sasanid period saw the rule of seven kings with Khusroe being regarded as the greatest of them and referred to as Malik al-Muluk (the king of kings). One of the seven kings was Harmazan, who ruled in Ahwaz. When the Muslims conquered Ahwaz, Harmazan was taken captive and brought before Umar, who said to him: "If you truly desire to remain safe, accept Islam or else I shall surely kill you."
Harmazan said: "Now that you shall eventually kill me, order for some water to be given to me for I am intensely thirsty."
Upon Umar's order some water was brought for him in a wooden bowl, but Harmazan said: "I shall not drink from this bowl for I am used to drinking water in bowls studded with jewels."
At this point the commander of the faithful(a.s) said: "This is not an extravagant request. Bring for him a crystalline bowl."
Water was poured into a crystalline bowl and presented to him but, instead of drinking it, he kept holding the bowl in his hands.
Umar said: "I have taken a covenant with Allah that I shall not kill you till you have drunk the water."
Having heard this, Harmazan smashed the bowl on the ground and spilled all the water in the process.
Umar stood aghast over this deception and, turning to the commander of the faithful(a.s), asked: "What should be done now?"
He (a.s) replied: "Since you had made his death contingent to his drinking the water and had entered into a covenant (with Allah) in this regard you cannot put him to death now; impose upon him the jizyah (tribute), instead."
Harmazan said: "I refuse to pay the tribute; however, now with serenity of mind and without any fear, I shall turn into a Muslim."
Saying this, he testified to the Unity of Allah and the Prophethood of the Noble Prophet (s.a.w) and became a Muslim.
Umar was delighted and seating Harmazan beside himself, he granted him a house in Madinah and allotted for him an annual sum of ten thousand dinars (from the public treasury).
3 – The Oath of Fudhul
Twenty years before the Noble Prophet's (s.a.w) proclamation of Prophethood and at a time when he (s.a.w) was exactly twenty years of age, an incident transpired as follows: One day, a person from the tribe of Bani Zubaid sold some goods to 'as Ibn Wail. 'as took possession of the goods but refused to pay him the money for it. In desperation, the man climbed atop the mountain of Abu Qubais and shouted: "O' People! Rush to the help of an oppressed one, who happens to be far from his tribe and relatives. Surely, reverence befits one, who possesses integrity, whereas there is no esteem for a cheat."
The people, who were around the Kabah, were inspired and motivated by this speech and so a group, consisting of some individuals from various tribes, gathered in the house of 'Abdullah Ibn Juda'an and made a pact amongst themselves to provide assistance to the oppressed ones and prevent anyone from being oppressed in Mecca. The Noble Prophet (s.a.w) too was a party to this agreement. Later, all of them set out and handed the person his money.
Years later, after the Noble Prophet (s.a.w) had proclaimed his Prophethood, he (s.a.w) said: "In the house of 'Abdullah Ibn Juda'an I had participated in a covenant such that had I been invited to the like of it in Islam, I would have surely accepted."
4 – Standing up for Islam until the End
Anas Ibn Nadhr was the uncle of Anas Ibn Malik - the slave of the Noble Prophet (s.a.w). As he had not participated in the battle of Badr, he said to the Noble Prophet (s.a.w): "O' Prophet of Allah! I could not participate in the battle that had loomed up before you nonetheless, I promise that should there ensue another battle, I shall surely participate in it."
When the battle of Uhud came to pass, he presented himself and fought in it. In the course of the battle when rumours were making round amongst the Muslim forces that the Noble Prophet (s.a.w) had been killed, some said: "O' How we wish we had a representative, whom we could send to 'Abdullah Ibn Ubayy, (the leader of the hypocrites), so that he could procure a pardon for us from Abu Sufiyan."
Some sat at a distance, hand upon hand, and worried and anxious, wondering about what would happen in the future.
Some others said: "Now that Muhammad has been killed, revert back to your original faith."
Hearing these words, Anas Ibn Nadhr announced: "I absolve myself from what these people recommend." He then asserted: "If Muhammad has been killed, the Allah of Muhammad is still alive. What is the purpose of life now that Noble Prophet has died? Continue fighting for the very purpose that the Noble Prophet fought!"
Then, unsheathing his sword and in accordance with his covenant, he battled with the enemies till he tasted martyrdom, suffering around eighty injuries of arrows and lances. His wounds were so numerous that when his sister Rabi' arrived, she managed to identify him only by means of his fingertips.
5 – The Muslim Slave
Fudhail Ibn Zaid Raqqashi, one of the Muslim officials, along with his soldiers laid siege to a fort, by the name of Shahriyaj, with the objective of conquering it. After some hours of battle, he returned to his camp for some rest.
In those days, slaves who came into the captivity of the Muslims, were put up for sale in the market. If they happened to be Muslims, they would battle against the enemies alongside their Muslim brethren.
In Fudhail's army too there was a slave soldier, who happened to lag behind the main contingent. Observing him to be alone, the enemy forces spoke to him in the local dialect from atop the fort and sought pardon and safety from him; the soldier acceded to their demand and granted them the letter of pardon.
When the forces of Islam advanced upon the fort, the enemy forces opened its door before them leaving the Muslim soldiers bewildered. The enemy forces carried the letter of pardon of the slave soldier and presented it to the Muslim army. Acknowledging a pardon granted by a slave soldier was very unusual in those days and so, the issue was referred to the second Caliph, who wrote back stating: "The Muslim slave is also from the Muslims and his covenants deserve the same respect as yours. Honour his letter of pardon and ensure that it is implemented."
 Suratul Nahl (16), Verse 91
 Shaytan, vol. 2, pg. 294
 Shanidaniha-e-Tarikh, pg. 290; Mahajjah al-Baidha, vol. 5, pg. 338
 Pand-e-Tarikh, vol. 2, pg. 42; Al-Kalam Yajurru al-Kalam
 Dastan-ha-e-Zindagi-e-Payambar, pg, 36; Tabaqat al-Kubra, vol. 1, pg. 128
 Paighambar Wa Yaran, vol. 1, pg. 334
 Dastan-ha-e-Ma, vol. 1, pg. 111; Kudak-e-Falsafi, vol. 2, pg. 17