"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).'" (Using translation from Nasāʾī 291)  

عَنْ حَجَّةِ النَّبِيِّ ﷺ فَحَدَّثَنَا أَنَّ رَسُولَ اللهِ ﷺ خَرَجَ لِخَمْسٍ بَقِينَ مِنْ ذِي الْقَعْدَةِ وَخَرَجْنَا مَعَهُ حَتَّى أَتَى ذَا الْحُلَيْفَةِ وَلَدَتْ أَسْمَاءُ بِنْتُ عُمَيْسٍ بِمُحَمَّدِ بْنِ أَبِي بَكْرٍ فَأَرْسَلَتْ إِلَى رَسُولِ اللهِ ﷺ كَيْفَ أَصْنَعُ؟ قَالَ «اغْتَسِلِي وَاسْتَثْفِرِي ثُمَّ أَهِلِّي»