Sahih Muslim
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Book Number: 035 / Hadith Number: 6523
Al-Agharr al-Muzani who was from amongst the Companions of Allah's Apostle (may peace be upon him) reported that Ibn 'Umar stated to him that Allah's Messenger (may peace 'be upon him) said: O people, seek repentance from Allah. Verily, I seek repentance from Him a hundred times a day.