Musannaf Ibn Abi Shayba 26:176

Waki’ narrated to us, from Sufyan, from Abdullah bin Khalid, from Abd al-Rahman bin Ma’qil, who said: I was with Ibn Abbas, and a man asked him about the grandfather. Ibn Abbas said to him, "Which father of yours is the greatest?" The man did not know what to say, so I said, "Adam." Ibn Abbas said, "Indeed, Allah says: 'O children of Adam.'"