It is narrated from Urwah bin Zubair (may Allah be pleased with them both) that the Prophet (ﷺ) instructed Sahla bint Suhail (may Allah be pleased with her) to breastfeed Salim five times so that he would become her mahram.
Hadith Referenceمسند الإمام الشافعي / كتاب النكاح / 1188