Musannaf Ibn Abi Shayba 21:675

Ya'la ibn 'Ubayd narrated to us, from Musa al-Juhani, who said: I asked al-Sha'bi, al-Hakam, 'Ata', and Mujahid about the prayer (supplication) being something fixed/timed. They said: "No, you are merely an intercessor, so intercede with the best of what you know."