Sahih Muslim Book 016, Hadith Number 4127.

Chapter : Al-Qasamah.

Sulaiman b. Yasar, the freed slave of Maimuna, the wife of Allah's Apostle (may peace be upon him), narrated from one of the Ansari Companions of Allah's Messenger (may peace be upon him) that Allah's Messenger (may peace be upon him) retained (the practice) of Qasama as it was in the pre-Islamic days.