Sahih Muslim Book 13, Hadith Number 4002.

Chapter : The deceased is entitled to reward for the sadaqa given after his death.

‘Aisha (Allah be pleased with her) reported that a man said to Allah’s Apostle (may peace be upon him): My mother died all of a sudden, and I think if she (could have the opportunity) to speak she would have (made a will) regarding Sadaqa’. Will I be entitled to reward if I give charity on her behalf? He (the Holy Prophet) said: Yes.