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'."
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."