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.
USC-MSA web (English) reference: Book 20, Hadith 2
Arabic reference: Book 20, Hadith 709