Musannaf Ibn Abi Shayba 3:2114

Ma'n ibn 'Isa narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, regarding the man who becomes forgetful in prayer: If he has straightened up standing, then upon him are the two prostrations. And if he remembers before he straightens up standing, then there is no forgetfulness [prostration] upon him.