Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 2821
Ghunaim b. Qais said: I asked Sa'd b. Abu Waqqas (Allah be pleased with him) about Mut'a, whereupon he said: We did that, and it was the day when he was an unbeliever living in (one of the) houses of Mecca.