Ja'far bin Muhammad said:
"My father told me: 'We cam to jabir bin 'Abdullah and asked him about the hajj of the prophet (ﷺ). He narrated: 'The Messenger of Allah (ﷺ) set out when there were five (days) remaining in Dhul-Qa'dah, and we set out with him. When he came to Dhul-Hulaifah, Asma' bint 'Umais gave birth to Muhammad bin Abi Bakr. She sent word to the Messenger of Allah (ﷺ) asking what she should do. He said: 'Perform Ghusl, bind yourself with a cloth then begin (the Talbiyah for Ihram).'" (DE)
English reference (DE): Vol. 1, Book 1, Hadith 292 (DE)
Arabic reference (DE): Book 1, Hadith 293 (DE)