Abu Dawud Book 2, Hadith Number 1510.
Chapter : On seeking pardon and repentance.
Narated By Al-Agharr al-Muzani : (Musaddad in his version said he was a companion of the Prophet)The Apostle od Allah (PBUH) said: My heart is invaded by unmindfullness, and I ask Allah’s pardon a hundred times in the day.