Ibn `Umar (May Allah be pleased with them) said: We counted Messenger’s saying a hundred times during one single sitting:
Ibn Mas`ud (May Allah be pleased with him) said: The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, “He who says:
‘Astaghfirullah al-lathee laa ‘ilaaha ‘illaa Huwal Hayyul Qayyoomu wa ‘atoobu ‘ilayhi.
‘I seek the forgiveness of Allah, Whom there is none worthy of worship except Him, the Living, the Self-Subsisting and Supporter of all, and I turn to Him in repentance.’
his sins will be forgiven even if he should have run away from the battlefield (while he was engaged in fighting for the Cause of Allah).”
[Abu Dawud, At-Tirmidhi and Al-Hakim (on conditions of Al-Bukhari and Muslim for accepting Hadith)].
– Allaah’s Messenger (Sallalaahu alaihi wa sallam) said:
“By Allaah, I seek the forgiveness of Allaah, and repent to Him more than seventy times in a day.”
“O poeple, repent to Allaah, for I verily repent to Him one hundred times a day.” 
“It is a heavy thing for my heart if I do not seek Allaah’s forgiveness a hundred times a day.”
 (Al-Bukhaari, cf. Al-Asqalaani, Fathul-Baari 11/101.]
 (Muslim 4/2076)
 Muslim 4/2075. Ibn ‘Athir explains that the prophet (peace be upon him) was always vigilant in his remembrance and drawing near to Allaah, and if he forgot to do any of what he normally did from time to time, or it slipped his mind, he felt as if he had wronged himself and so he would begin to seek the forgiveness of Allaah. See Jaami’ul-‘Usool 4/386.