From Sahih Muslim (Hadith 4220):
A'isha (Allah be pleased with her) reported that a man said to Allah's Apostle (ﷺ):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
Reference: Muslim 4220