Musannaf Ibn Abi Shayba 3:3672

Abbad ibn al-Awwam narrated to us, from Hisham, from al-Hasan, regarding the pregnant woman who sees blood: He said: "If she sees it as she used to see it before that in her menstrual cycles, she shall abandon the prayer. But if it is only for a day or two, she shall not abandon the prayer."