Musannaf Ibn Abi Shayba 9:1549

Mu'tamir ibn Sulayman narrated to us, from his father, from al-Hasan, who said: The waiting period for women is forty days, then she performs ghusl and prays. And al-Sha'bi said: The waiting period is two months, then she is in the status of a woman with irregular vaginal bleeding (mustahada).