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: 6295
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: I have held covenant with Thee which Thou wouldst not break, so for any believer whom I curse or beat, make that an expiation on the Day of Resurrection.