Musannaf Ibn Abi Shayba 9:1063

Muhammad ibn Abi ‘Adi narrated to us, from Humayd, from Bakr ibn ‘Abd Allah, who said: It reached me that al-Hajjaj said: "If she is in doubt regarding her menstruation, she shall perform ghusl and pray, and he shall not approach her until she becomes pure." He (Bakr) said: "How evil is what he said! Prayer is of greater sanctity."