Sahih Muslim Book 25, Hadith 19
This hadith has been narrated on the authority of Hisham b. 'Urwa with the same chain of transmitters.
Allah's Messenger, my mother died all of a sudden without making any will. I think if (she could have the opportunity) to speak she would have made a Sadaqa. Would there be any reward for her if I give charity on her behalf? He (the Holy Prophet) said: Yes.
When a man dies, his acts come to an end, but three, recurring charity, or knowledge (by which people) benefit, or a pious son, who prays for him (for the deceased).