Chapter 30: The Book of Wills
It was narrated from Sa' d bin 'Ubadah that his mother died. He said: "O Messenger of Allah, my mother has died; can I give charity on her behalf?" He said: "Yes." He said: "What kind of charity is best?" He said: "Providing drinking water." And that is the drinking-fountain of Sa' d in Al-Madinah.
Sunan an-Nasa'i 3666