Musannaf Ibn Abi Shayba 3:1255

Ibn Fudayl narrated to us, from Abu Sufyan al-Sa'di, from Abu Nadra, from Abu Sa'id, from the Prophet (peace be upon him), who said: "There is no prayer for the one who does not recite 'Al-Hamdulillah' (Surah al-Fatiha) and a Surah in every rak'ah, whether it is in the obligatory prayer or otherwise."