Musnad Ahmad 920:7

'Abdullah narrated to us, 'Amr al-Naqid narrated to us, Sufyan narrated to us, from 'Amr, from Sa'id ibn Jubayr, from Ibn 'Abbas, from Ubayy ibn Ka'b, from the Prophet, peace and blessings of Allah be upon him, who said: "If I had wished, I could have taken a wage for it" [Al-Kahf: 77].