From Sahih Muslim (Hadith 4350):
Sulaiman b. Yasar, the freed slave of Maimuna, the wife of Allah's Apostle (ﷺ), narrated from one of the Ansari Companions of Allah's Messenger (ﷺ) that Allah's Messenger (ﷺ) retained (the practice) of Qasama as it was in the pre-Islamic days
Reference: Muslim 4350