Musnad Abu Ya'la al-Mawsili 1:52

Narrated to us Ubayd Allah ibn Umar, narrated to us Abd al-Rahman ibn Mahdi, from Malik ibn Anas, from Abd al-Rahman ibn al-Qasim, from his father, from Asma bint Umays, that she gave birth at Dhu al-Hulayfah. So Abu Bakr asked the Prophet, peace and blessings of Allah be upon him, and he said: "Command her to perform ghusl (ritual bath) and to enter into ihram."