Musnad Ahmad 27:1440

Musa—meaning Ibn Dawud—narrated to us, Ibn Lahi'ah narrated to us, from Humayd ibn Hani', from 'Abbas ibn Julayd al-Hajari, from Ibn 'Umar, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, how many times should a slave be forgiven?" He remained silent toward him, then he repeated it, and he remained silent toward him, then he repeated it. He then said: "Forgive him seventy times every day."