From Sahih Muslim (Hadith 2908):
A'isha (Allah be pleased with her) reported that Asma' bint `Umais gave birth to Muhammad b Abu Bakr near Dhu'l-Hulaifa. The Messenger of Allah (ﷺ) commanded Abu Bakr to convey to her that she should take a bath and then enter into the state of Ihram
Reference: Muslim 2908