Musannaf Ibn Abi Shayba 3:2056

Abu Abd al-Rahman al-Muqri narrated to us, he said: Sa'id ibn Abi Ayyub narrated to us, he said: Abu Aqil narrated to me that he heard Sa'id ibn al-Musayyib saying: The two prostrations of forgetfulness in supererogatory prayers are like the two prostrations of forgetfulness in the obligatory prayers.