Maliks Muwatta Book 020, Hadith Number 002.

Section : The Ghusl to Enter Ihram.

Yahya related to me from Malik from Yahya ibn Said from Said ibn al-Musayyab that Asma bint Umays gave birth to Muhammad ibn Abi Bakr at Dhu'l-Hulayfa and Abu Bakr told her to do ghusl and then enter ihram.