Book 20, Number 20.1.1:

Yahya related to me from Malik from Abd ar-Rahman ibn al-Qasim from his father from Asma bint Umays that she gave birth to Muhammad ibn Abi Bakr at al-Bayda. Abu Bakr mentioned this to the Messenger of Allah, may Allah bless him and grant him peace, and he said, "Tell her to do ghusl and then enter ihram."