The Book of Wills
كتاب الوصايا
Chapter 8: The Virtue Of Charity Given On Behalf Of The Deceased
It was narrated from Sa'd bin 'Ubadah that he consulted the Prophet about a vow which his mother had to fulfill, but she died before doing so. The Messenger of Allah said
"Fulfill it on her behalf."
Chapter 9: Mentioning The Different Reports From Sufyan
It was narrated that Sa'd bin 'Ubadah said
"I said: 'O Messenger of Allah, my mother has died; shall I give in charity on her behalf?' He said: 'Yes.' I said: 'What kind of charity is best?' He said: 'Providing drinking water.'"