Al-Adab Al-Mufrad #291

Umm ad-Darda' said, "Abu'd-Darda' stood up in the night to pray. He was weeping and said, 'O Allah! You made my physical form good, so make my character good!' until morning. I said, 'Abu'd-Darda', your only supplication for the entire night was for good character!' He replied, 'Umm ad-Darda', the Muslim perfects his character until good character enters him into Paradise; and taints his character until his bad character enter him into Hellfire. The Muslim is forgiven while he is asleep.' I asked, 'Abu'd-Darda', how can be forgiven while he is asleep?' He said, 'His brother arises in the night, performs the night prayer, and supplicates to Allah Almighty and is answered. He supplicates for his Muslim brother and his supplication is answered.'"