Musannaf Ibn Abi Shayba 3:1244

Ibn ‘Ulayyah narrated to us, from al-Walid ibn Abi Hisham, from Wahb ibn Kaysan, who said: Jabir ibn ‘Abd Allah said: "Whoever does not recite the Mother of the Qur’an (al-Fatiha) in every rak’ah, then he has not prayed, except if he is behind the Imam."