Yahya related to me from Malik from Yahya ibn Said from Said ibnal-Musayyab that Asma bint Umays gave birth to Muhammad ibn Abi Bakrat Dhu'l-Hulayfa and Abu Bakr told her to do ghusl and then enterihram. (PT)
USC-MSA web (English) reference: Book 20, Hadith 2 (PT)
Arabic reference (PT): Book 20, Hadith 709 (PT)