Abu’d-Dardaa’ (may Allaah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said:

“There is no Muslim who prays for his brother in his absence, but the angel will say: And you will have something similar.”

Narrated by Muslim (2732).

Al-Qaadi ‘Iyaad (may Allah have mercy on him) said:

He will have a reward commensurate with what he prayed for, because even though he prayed for someone else.

In fact, he did two righteous deeds, the first of which was remembering Allah (may He be exalted) and showing devotion to Him alone, turning to Him in his words and in his heart.

The second was loving good for his Muslim brother and praying for him, which is doing good for a Muslim, and he will be rewarded for that.

The hadith states that the du‘aa’ will be answered.

End quote from Ikmaal al-Mu‘allim (8/228).
Source: IslamQA