The Messenger of Allah (sal Allahu alaihi wa sallam) said: “The angel on the left holds up his pen (refrains from writing down) for six hours after a Muslim commits a sin. If the person regrets it and asks Allah for forgiveness, he casts it aside (does not write it down), otherwise he writes it down as one (sayi’ah).” [Mu’jam at-Tabaraani al-Kabeer]
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This is authentic.