It was narrated from Jabir bin 'Abdullah, in the Hadith of Asma' bint 'Umair, when she gave birth in Dhul-Hulaifah, that the Messenger of Allah (ﷺ) said to Abu Bakr:
"Tell her to perform Ghusl and enter Ihram." (RU)
English reference (RU): Vol. 1, Book 1, Hadith 137 (RU)
Arabic reference (RU): Book 1, Hadith 216 (RU)