Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6289
This hadith has been transmitted on the authority of A'mash and in the hadith transmitted on the authority of 'Isa the words are: Make it a source of reward, and in the hadith transmitted on the authority of Abu Huraira (the words are):" Make it a source of mercy."