Yahya related to me from Malik from Abd ar-Rahman ibn al-Qasimfrom his father from Asma bint Umays that she gave birth to Muhammadibn Abi Bakr at al-Bayda. Abu Bakr mentioned this to the Messenger ofAllah, may Allah bless him and grant him peace, and he said, "Tell herto do ghusl and then enter ihram."
USC-MSA web (English) reference: Book 20, Hadith 1
Arabic reference: Book 20, Hadith 708