Allah, the Wise, has said:
وَ مَا خَلَقْتُ الْجِنَّ وَ الإِنْسَ إِلاَّ لِيَعبُدُونَ
"And I have not created the jinn and the men except that they should serve Me." 
Imam Sajjad (a.s) said:
مَنْ عَمِلَ بِمَا افْتَرَضَ اللهُ فَهُوَ مِنْ أَعْبَدِ النَّاسِ.
"One, who performs (all) that Allah has made obligatory upon him, is the most worshipping of all people." 
A Mu'min performs the obligatory and the recommended acts for these are the basis for Allah's obedience and subservience. If one performs these, it is as if he has exhibited his devotion and subservience in entirety.
The best worship is that which is protected from external influences and shielded from internal catastrophes.
Should the deeds of a person be continual and flawless, the person shall be successful in his worship, however meagre they might be.
Those who seek plain knowledge and mere external excellences but remain heedless of the soul and the reality of worship, do not achieve anything except a mould, as far as the worship of the Beneficent Allah is concerned.
1 – The Outcome of ‘Dry’ Worship
The Kharijites were individuals who as a result of their fanaticism and extremism, had gone greatly astray. Their leader was a person by the name of Hurqus Ibn Zuhair, who, during the time of the Noble Prophet (s.a.w), had drowned himself in prayers, fasts and other acts of worship causing many of the Muslims to become fascinated with him.
When the Noble Prophet (s.a.w) had been distributing the spoils of war after the battle of Hunain, this person, who was just an arid and ceremonial worshipper, said to him with great impudence: "O' Muhammad! Conduct yourself with fairness and equality," and repeated the sentence three times.
When he had repeated it for the third time the Noble Prophet (s.a.w) became greatly disturbed and said: "If I do not behave with fairness and equality, then who shall behave so?"
This 'dry' worshipper eventually set out for battle against Imam Ali (a.s) in the battle of Nahrawan and was killed. When the Imam's (a.s) eyes fell upon his inauspicious corpse lying amongst the dead, he (a.s) offered a prostration of thanksgiving and said (to his companions): "You have killed the worst of the men."
2 – Worship out of Love
Sadi recounts: "During one of my travels to Mecca, I found myself in the company of a group of pure-hearted and sincere youths, who used to chant supplications and recite poems befitting men of letters and engage themselves in acts of worship with an extraordinary presence of heart.
On the way we were joined by a 'dry' and 'soulless' worshipper, who did not approve of this mystical state of theirs and, being unaware of the inner fervour of the devoted youths, persistently criticized their behaviour.
We continued on our way till we reached a place known as Bani Hilal where, a black-faced Arab child approached us and began chanting for us in such a manner so melodious that even enraptured the birds and caused them to descend from their flight.
The enchanting melody instilled such fervour in the worshipper's camel that it flung the worshipper to the ground and frantically rushed out into the desert.
Turning to the worshipper, I said: "O' Old worshipper! You observe how a pleasant melody tends to affect even the animals whereas you are apathetic and indifferent (remaining unaffected by the spiritual melodies, not submitting your heart to Allah and not acquiring purity, unlike the pious and the pure-hearted ones).""
3 – The Long Worshipper
Owais Qarani was of those individuals, who had become completely enraptured with worship such that, at times, he would pass the entire night in the state of ruku while at other times he would say: "Tonight is the night for prostration," and would remain in prostration till dawn.
He was told: "What is this inconvenience that you impose upon yourself?" He replied: "O' how I wish that the entire eternity were one night, which I could pass in one prostration."
Rabi' Ibn Khathim (buried in Mashhad and popularly known as Khwajah Rabi') relates: "I was in Kufah and my prime objective was to meet Owais Qarani. When I eventually found him, he was engaged in his afternoon prayers near the river Euphrates. I said to myself: "I shall wait till he completes his prayers."
Completing his afternoon prayers, he began reciting supplications and continued to do so till it became time for the Maghreb and Isha prayers. Having offered them, he engaged himself in offering the recommended prayers - at times in ruku' while at other times in sajdah, and this continued till the night reached its termination. He offered his morning prayers and once again engaged himself in reciting supplications till the sun rose above the horizon whereupon he permitted himself some rest. Waking up, he performed his ablution and was about to start his worship when I approached him and said: "How greatly you trouble yourself!" He said: "It is for achieving comfort that I endure such effort." I said to him: "I did not see you eat anything. How do you manage to arrange for your expenses?" He said: "Allah has taken upon Himself to provide sustenance to His servants. Now do not indulge in any more of such talks. Having spoken thus, he left.""
4 –Iblis’ Worship
The Commander of the Faithful (a.s) said: "Take lesson from Allah's action with respect to Iblis, for He annulled all his acts of worship (because of his arrogance); he had worshipped Allah for six thousand years, and you know not if they were of the years of the world or those of the hereafter (every day of which is equivalent to fifty thousand years of this world). And this was a consequence of a moment's disobedience (in that he considered himself to be superior to Adam and refused to prostrate before him). So, after the Satan, is there anyone who can remain safe from Allah's punishment by being disobedient to Him?"
Imam Sadiq (a.s) was asked: "For what reason did Allah respite Iblis till the 'appointed time'?"
The Imam (a.s) replied: "Because of his praise and thanksgiving to Allah."
He (a.s) was asked again: "What was his praise and thanksgiving?"
He (a.s) replied: "His six thousand years of worship in the heavens." (On another occasion he (a.s) said: "He offered, in the seven heavens, a two-rak'at prayer which extended for a period of six thousand years)."
5 – Imam Sajjad (a.s)
The reason for Imam Sajjad (a.s) being given the title of Zainul 'abidin was that one night, as he stood for prayers in his place of worship, Satan manifested himself in the form of a huge serpent with the intention of distracting him (a.s) from his prayers. But when the Imam (a.s) took no notice of him, the Satan approached him (a.s) and bit his big toe causing pain to run through him, but despite this act of his, the Imam (a.s) continued to remain oblivious of him and carried on with his prayers.
When, after the completion of his (a.s) prayers, he realized that it was Satan, he cursed him saying: "O' Accursed! Go away," and once again engaged himself in worship. It was at this moment that he (a.s) heard an angel call out three times: "You are the embellishment of the worshippers."
 Suratul Dhariyat (51), Verse 56
 Shaytan, vol. 2, pg. 113
 Tadhkeratul Haqa`iq, pg. 28
 Dastan-ha Wa Pand-ha, vol. 9, pg. 77; ~Ali Wa Farzandan (Dr. Taha Husain), pg. 123
 Hikayat-ha-e-Gulistan, pg. 128
 Paighambar Wa Yaran, vol. 1, pg. 350; Naseekh al-Tawarikh – ~Ali , pg. 176
 Nahj al-Balaghah, (Faidh al-Islam), pg. 780, Sermon 234
 Iblis Nameh, pg. 165; ~Ilal al-Sharai~, vol. 2, pg. 243
 Muntahal A~mal, vol. 2, pg. 3