Sahih Muslim Book 013, Hadith Number 4003.
Chapter : The deceased is entitled to reward for the sadaqa given after his death.
'Aisha (Allah be pleased with her) reported that a man came to Allah's Apostle (may peace be upon him) and said: 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.