Umm ad-Dardaa’ said, “Abud-Dardaa’ stood up one night to pray. He was weeping while saying, ‘O Allah! You perfected my physical form, so perfect my character!’ until morning.
I said, ‘O Abud-Dardaa’, your only supplication for the entire night was for good character!’
He replied, ‘O Umm ad-Dardaa’- the Muslim perfects his character until his good character takes him to Paradise. And he corrupts his character until his bad character takes him to the Fire. The Muslim is forgiven while he is asleep.’
I asked, ‘O Abud-Dardaa’, how can a Muslim be forgiven while he is asleep?’
He said, ‘His brother wakes up at night and performs tahajjud and supplicates and is answered. He supplicates for his Muslim brother and his request is answered.'”
[al-Adab al-Mufrad , #290]
This is in accordance with the hadith where the prophet صلى الله عليه وسلم said,
“The supplication of a Muslim for his brother in his absence will certainly be answered. Everytime he makes a supplication for good for his brother, the angel appointed for this particular task says: `Ameen! May it be for you, too’.” [Muslim].