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."
Muwatta Malik Book 20, Hadith 1