"The Messenger of Allaah (sal-Allaahu 'alayhe wa sallam) said: "By Allaah, I seek forgiveness and repent to Allaah, more than seventy times a day."  He (sal-Allaahu 'alayhe wa sallam) also said: "O People, Repent to Allaah! Verily I repent to Him a hundred times a day."  He (sal-Allaahu 'alayhe wa sallam) also said: "Whoever says:
أَسْتَغْفِرُ اللَّهَ الْعَظيمَ الَّذِي لاَ إِلَهَ إِلاَّ هُوَ الْحَيُّ القَيُّومُ وَأَتُوبُ إِلَيهِ
"I seek Allaah"s forgiveness, besides whom, none has the right to be worshipped except He, The Ever Living, The Self-Subsisting and Supporter of all, and I turn to Him in repentance." 
Astaghfirullaahal-'Adheemal-lathee laa 'ilaaha 'illaa Huwal-Hayyul-Qayyoomu wa 'atoobu 'ilayhi.
 Al-Bukhari, cf. Al-Asqalani, Fathul-BAri 11/101 No. 6307  Muslim 4/2076 No. 2702  Abu Dawud 2/85, At-Tirmithi 5/569, and Al-Hakim who declared it authentic and Ath-Thahabi agreed with him 1/511. Al-Albani graded it authentic in Sahih At-Tirmithi 3/182. See also Jami'ul-'Usool li-'Ahdaith Ar-Rasool 4/ 389-90 checked by Al-Arna'ut.  At-Tirmithi, An-Nasa'i 1/279 and Al-Hakim. See also Al-Albani, Sahih At-Tirmithi 3/183, and Jdmi'ul-'Usool with Al-Arna'ut's checking 4/144.  Muslim 1/350.  Muslim 4/2075 No. 2702  Al-Bukhari 7/150