Musnad Ahmad 27:905

Ahmad ibn Abd al-Malik narrated to us, Zuhayr informed us, Abu Ishaq narrated to us, from Mujahid, from Ibn Umar, who said: I was sitting with the Prophet, peace and blessings of Allah be upon him, and I heard him seek forgiveness one hundred times. Then he said: "O Allah, forgive me, have mercy upon me, and accept my repentance; indeed, You are the Accepting of Repentance, the Merciful," or "Indeed, You are the Accepting of Repentance, the Forgiving."