It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The angel on the left holds back the pen for six hours from the Muslim servant who errs or does wrong; if, during that time, he feels remorse and seeks forgiveness from Allah, he (the angel) discards it (and does not write it), otherwise it is written as one (sin)."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7734