Musannaf Ibn Abi Shayba 3:6024

Abu Usama narrated to us, from Ibn 'Awn, from Muhammad, from Kathir ibn Aflah, who said: We were in Medina in the house of Abu Yusuf, occupied with some accounts of ours that we were calculating, and Zayd ibn Thabit was with us. He said: "A man's prayer with the Imam is multiplied over his prayer alone by twenty-some degrees."