Majma al-Zawaid — Hadith 17576

Sayyiduna Abu Umamah (may Allah be pleased with him) transmits this saying of the Prophet (peace and blessings be upon him): "The angel on the left keeps the pen raised for six hours from the Muslim servant who has erred (the narrator is unsure, or perhaps the words are:) who has done evil (that is, for six hours he does not record his evil deed). If that servant feels remorse and seeks forgiveness from Allah Almighty for that sin, he does not record that sin; otherwise he records it as one sin."
Al-Tabarani recorded it with various chains, and the narrators of one of them have been declared trustworthy.