Sahih Muslim Book 7, Hadith Number 2762.

Chapter : Donning of Ihram for women who are in the state of menses and parturition and excellence of bath while entering into the state of Ihram.

‘Aisha (Allah be pleased with her) reported that Asma’ hint ‘Umais gave birth to Muhammad b Abu Bakr near Dhu’I-Hulaifa. The Messenger of Allah (may peace be upon him) commanded Abu Bakr to convey to her that she should take a bath and then enter into the state of Ihram.