Musnad Abu Ya'la al-Mawsili 111:49

Muhriz ibn 'Awn narrated to us, he said: Ibrahim ibn Sa'd ibn 'Abd al-Rahman ibn 'Awf narrated to us, from his father, from Talhah ibn 'Abd Allah ibn 'Awf, who said: I prayed behind Ibn 'Abbas over a funeral, and he recited the Opening of the Book (al-Fatihah) and a surah, and he recited aloud until he made us hear. When he finished, I took his hand and asked him about that. He said: "It is a Sunnah and a truth."