Musannaf Ibn Abi Shayba 3:2106

Al-Ala’ ibn Mansur narrated to us, he said: Al-Haytham ibn Humayd informed us, from Ubayd Allah ibn Ubayd, from Zuhayr al-Homsi, from Thawban, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "For every forgetfulness, there are two prostrations."