Hadith about Esal-e-Sawab
Hadrat Sayyiduna Saad Bin Ubadah asked, "Ya Rasulullah (May Peace and blessing of ALLAH be upon him)! My mother has passed away (I want to give some Sadaqah (charity) on behalf of her), which Sadaqah would be the best for her?" The Holy Prophet (May Peace and blessing of ALLAH be upon him) replied, "Water." So, Sayyiduna Saad got a well dug and said, "This is for Saad"s mother."
(Sunan Abu Dawood, pp. 1348, Hadis 1681)