Musannaf Ibn Abi Shayba 3:1627

Waki’ narrated to us, from Isra’il, from Jabir, from ‘Amir and al-Hakam, regarding a man who forgot the Opening of the Book (al-Fatiha). Al-Sha’bi said: He performs two prostrations of forgetfulness. And al-Hakam said: He recites it when he remembers it.