Sahih Muslim Book 009, Hadith Number 3569.
Chapter: Li'an (Invoking curse).
Abu Huraira (Allah be pleased with him) reported that Sa'd b. 'Ubada al-Ansari said: Messenger of Allah, tell the if a man finds his wife with another person, should he kill him? Allah's Messenger (may peace be upon him) said: No. Sa'd said: Why not? I swear by Him Who has honoured you with Truth. There upon Allah's Messenger (may peace be upon him) said: Listen to what your chief says.