Muwatta Malik 706

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّ أَسْمَاءَ بِنْتَ عُمَيْسٍ، وَلَدَتْ، مُحَمَّدَ بْنَ أَبِي بَكْرٍ بِذِي الْحُلَيْفَةِ فَأَمَرَهَا أَبُو بَكْرٍ أَنْ تَغْتَسِلَ، ثُمَّ تُهِلَّ ‏.‏

Yahya related to me from Malik from Yahya ibn Said from Said ibn al-Musayyab that Asma bint Umays gave birth to Muhammad ibn Abi Bakr at Dhu'l-Hulayfa and Abu Bakr told her to do ghusl and then enter ihram

Reference

Muwatta Malik 706

In-book

Book 20, Hadith 2