Sahih Bukhari Volume 2, Book 24, Hadith Number 546.

Narated By Zainab, : (The daughter of Um Salama) My mother said, “O Allah’s Apostle! Shall I receive a reward if I spend for the sustenance of Abu Salama’s offspring, and in fact they are also my sons?” The Prophet replied, “Spend on them and you will get a reward for what you spend on them.”