Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 251
The Excellence of Supplicating in one's Absence

Allah, the Exalted, says:

"And those who came after them say: `Our Rubb! Forgive us and our brethren who have preceded us in Faith.''' (59:10)

"And ask forgiveness for your sin, and also for (the sin of) believing men and believing women''. (47:19)

"Our Rubb! Forgive me and my parents, and (all) the believers on the Day when the reckoning will be established.'' (14: 41)

1494. Abud-Darda' (May Allah be pleased with him) reported: I heard the Messenger of Allah (PBUH) saying, "Whenever a Muslim supplicates for his (Muslim) brother in his absence, the angels say: `May the same be for you too'.''
[Muslim].

Commentary:  This Hadith makes it evident that one who prays for someone in his absence also stands to benefit from it because the angel appointed for the task of praying for those who pray for others will pray for him, saying, "O Allah! Grant him also the same which he has asked for others.''

1495. Abud-Darda' (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "The supplication of a Muslim for his (Muslim) 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: `A   meen! May it be for you, too'.''
[Muslim].