Musannaf Ibn Abi Shayba 9:1201

Ibn Idris narrated to us, from Mutarrif, from ‘Amr ibn Salim, who said: Ubayy ibn Ka‘b said: "O Messenger of Allah, a number of the numbers of women have not been mentioned in the Book of Allah: the young, the old, and those who are pregnant." So Allah, the Exalted, revealed: "And those who despair of menstruation from your women, if you are in doubt, their period is three months, and those who have not menstruated, and those who are pregnant, their term is until they lay down their burden."