Sahih Muslim 4220
صحيح مسلم#4220SahihA'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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 25:17