Musannaf Ibn Abi Shaybah

Hazrat Ibn Musayyib and Ibrahim state that the waiting period (istibra) for a slave woman who menstruates is two menstrual cycles.

Hazrat Ata says that a slave girl was brought to me whose istibra’ (waiting period) was being observed; after one menstrual cycle, her pregnancy became apparent. After that, they began to observe istibra’ through two menstrual cycles.