Musannaf Ibn Abi Shayba 3:3669

Isra'il narrated to us, from Ayyub, who said: I wrote to Nafi' asking him about a pregnant woman who sees blood. He wrote back to me: I asked Sulayman ibn Yasar about a woman who sees blood that is neither menstruation nor postpartum bleeding. He said: She should perform ghusl, bind herself with a cloth, and pray.