Sahih Muslim Book 5, Hadith Number 2195.

Chapter : Getting of reward to Sadaqa for the dead given on his or her behalf.

Asma’ bint Abu Bakr reported: My mother who was a polytheist came to me when he (the Holy Prophet) entered into treaty with, the Quraish (of Mecca). I inquired from the Messenger of Allah (may peace be upon him) saying: Messenger of Allah, there has come to me my mother and she is inclined; should I (in this state of her mind) show her kindness? He said: Yes, treat her kindly.