The most superior way of asking forgiveness from Allah
Allah Almighty says in Aya (110) of Surat Alnisa' {And whoever does evil or wrongs himself, then asks forgiveness of Allah, he will find Allah All-Forgiving, Most Merciful}, so Allah assures that whoever commits an evil deed, or wrongs himself with a great or small sin, then he repents to Allah, gives up the sins he committed, and asks Allah's forgiveness for them, then He will find Allah All-forgiving, Most Merciful for those who repent, believe, and do righteous deeds.
Seeking forgiveness of Allah is a great way to be safe from Allah's punishment in the worldly life as Allah says in Aya (33) of Surat Alanfal {But Allah would not punish them while You are among them, and Allah would not punish them while they seek forgiveness}, so the Muslims have two protections from Allah's punishment, the first was lost with the death of Allah's Messenger while the second will remain as long as they seek forgiveness from Allah.
Allah's Messenger, Muhammad (PBUH), used to seek forgiveness frequently, even though He had been forgiven for His past and future sins as mentioned in Aya (2) of Surat Alfath, as Allah's Messenger (PBUH) considered that neglecting, at times, the remembrance of Allah which He was accustomed to doing constantly, was a sin in relation to His exalted station, so he would seek forgiveness one hundred a day, as Al-Agharr Al-Muzani reported that Allah's Messenger (PBUH) said: "There is (at times) some sort of shade upon My heart, and I seek forgiveness from Allah a hundred times a day" (Sahih Muslim 2702).
The Messenger of Allah, (PBUH) used to urge His companions, May Allah be pleased with them, to constantly seek forgiveness of Allah, and various forms of seeking forgiveness have been narrated from Him, but He (PBUH) explained the best and most beloved form of seeking forgiveness to Allah Almighty, the most rewarding and the most likely to be accepted, and Allah's Messenger (PBUH) called it the master of forgiveness (Sayyid Al-Istighfar) because it encompasses all meanings of repentance, as Shaddad bin Aws reported that the Prophet (PBUH) said: "The most superior way of asking forgiveness from Allah is Allahumma anta rabbi la illaha illa anta, khalaqtani wa ana abduka, wa ana ala ahdika wa wa'dika ma-stata'tu, a'auzu beka min sharri ma sana'tu, abu'u laka be ne'matika alayya wa abu'u laka be-zanbi fa-ghfir lee fa innahu la yaghferu alzunuba illa anta: O Allah! You are my Lord, there is none worthy of worship but You. You created me and I am Your servant, I keep Your covenant, and my pledge to You so far as I am able, I seek refuge in You from the evil of what I have done, I admit to Your blessing upon me, and I admit to You my sin, so Forgive me for there is none who may forgive sins but You, the Messenger of Allah added, if somebody recites it during the day with firm faith in it, and dies on the same day before the evening, he will be from the people of Paradise, and if somebody recites it at night with firm faith in it, and dies before the morning, he will be from the people of Paradise" (Sahih Albukhari 6306).