Musannaf Ibn Abi Shayba 9:748

Sharik narrated to us, from Abd al-Malik, from Ata, who said: A slave woman was placed with me to undergo *istibra* (to establish the emptiness of her womb), and she had one menstrual cycle. Then it became apparent that she was pregnant. So he would perform *istibra* for her with two menstrual cycles.