Asking for Forgiveness of Allah

 فَاعْلَمْ أنَّهُ لا إِلَهَ إِلاّ اللَّهُ وَاسْتَغْفِرْ لِذَنْبِكَ وَلِلْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ وَاللَّهُ يَعْلَمُ مُتَقَلَّبَكُمْ وَمَثْوَاكُمْ
پس جان رکھو کہ خدا کے سوا کوئی معبود نہیں اور اپنے گناہوں کی معافی مانگو اور (اور) مومن مردوں اور مومن عورتوں کے لئے بھی۔ اور خدا تم لوگوں کے چلنے پھرنے اور ٹھیرنے سے واقف ہے
Therefore, know that there is no god but Allah and ask forgiveness for your sin, and also for [the sin of] believing men and believing women. And Allah knows well your moving about and your place of rest.
47:19
Exegesis:
The clause “ask forgiveness for your sin” is also attested elsewhere in the Holy Qur'an (40:55). With due consideration to the fact that all prophets are infallible, there are grounds for such asking for forgiveness:
Asking for forgiveness is a kind of worshipping God even if no sin has been committed.
It may ad hoc indicate avoidance from doing what is preferable (tark al-ūlā ). In other words, the Prophet (SAW) was supposed to do preferable acts but he abstained from doing them.
Asking for forgiveness may turn into a common practice for people.
It leads to attaining supreme stations by the Prophet (SAW).
It may connote asking for forgiveness for the sins committed by people in respect of the Prophet (SAW).
Asking for forgiveness is not at all times for one’s sins and faults, but perceiving that he is so inferior against Divine Glory and Greatness, man asks for forgiveness.
Addressing His Prophet (SAW) God Almighty says unto him: “know that there is no god but Allah and ask forgiveness for your sin and also for [the sin of] believing men and believing women.” Imamī scholars are unanimous in maintaining that prophets, particularly the Seal of the Prophets (SAW), are infallible. “Your sin” (dhanbika) may either indicate abstaining from doing preferable acts or preoccupation with mundane affairs. It is narrated from the Prophet (SAW) that he asked forgiveness of God seventy times a day. It was for his preoccupation with carrying out the tasks of promulgation of the Islamic faith and attending to the affairs of his prophethood. It is in the Qur'an that he is a man like other men (“Verily, I am a human being like you”). Owing to the restrictions of human nature, preoccupation with any matter may at times hinder one from attending fully to God. It is to be noted that his preoccupation with attending to the affairs of his prophethood was out of is obedience to God and acting upon His Command but “the good deeds of those disfavored by God are like the faults of favorites of God.” That is why he is asked to ask for forgiveness for himself and believers. In other words, he is asked to turn away from people following the communication of the Message and attend fully to the glorification of God. It is a source of hope to believers that the Noble Prophet (SAW) is asked to pray for their forgiveness. His prayer will not remain unanswered unless we really have faith in God and His Messenger. It is worthy of note that the Prophet (SAW) with his supreme station is required to ask for forgiveness let alone us since we commit sins and neglect our duties at all times. Asking for forgiveness of “the All-Forgiving of sins” and turning away from opposition to God lead to turning toward God and are regarded per se as an act of worship.
The Verse ends in “Allah knows well your moving about and your place of rest.” In other words, God is fully aware of the acts committed by man as well as his abode in the Hereafter. Past, present, and future make no difference to His Omniscience.
Narrating on the authority of Hudhayfa Yamānī, Tabarsī records that the former said unto the Prophet (SAW) that he  had a sharp tongue and as a consequence was afraid of ending up in Hell for that sake. The Noble Prophet (SAW) asked him why he was not asking for forgiveness as he [the Prophet (SAW)] was asking for the same one hundred times a day.
According to a Prophetic tradition, uttering “I ask forgiveness of God” and “There is no god but Allah” are the best devotions since God Almighty says in a Qur'anic Verse: “Therefore know that there is no god but Allah and ask forgiveness for your sin, and also for [the sin of] believing men and believing women.” [1]
It is to be noted that asking for forgiveness of God may not be solely done verbally but true repentance lies in being regretful from committing sins and intending to not turn to sins and mere praying for forgiveness is of no avail. According to a Prophetic tradition recurrently narrated “Regret [from committing sins] is asking for forgiveness,” in other words, regret from committing sins forms the basis of asking for forgiveness of God. However, such regret necessitates intending to abstain from committing further sins. It may be said that true asking for forgiveness lies in repentance and intention as to committing no further sins; nonetheless, praying for forgiveness and other requirements attested in traditions and narrations are the conditions of perfect repentance. The repentant ask for forgiveness of the All-Forgiving of sins by heart and tongue.
[1] Manhaj al-Sādiqīn.