Musnad Ahmad

It is narrated from Hazrat Sa'd bin Ubadah (may Allah be pleased with him) that his mother passed away, so he submitted to the Holy Prophet (peace and blessings be upon him), "O Messenger of Allah! My mother has passed away. Can I give charity on her behalf?" The Holy Prophet (peace and blessings be upon him) said, "Yes!" He asked, "Then which charity is the best?" The Holy Prophet (peace and blessings be upon him) said, "Providing water to drink." The narrator says that this is the real reason for the family of Sa'd providing water in Madinah Munawwarah.