Musannaf Ibn Abi Shayba 3:652

Yahya ibn Sa'id al-Qattan narrated to us, from Uthman ibn al-Aswad, who said: I said to Mujahid, "Should I supplicate for myself during the obligatory prayer?" He said, "Do not supplicate for yourself until you have performed the tashahhud." He said: And I asked Ata', and he said, "You should be cautious by [limiting your supplication to] seeking forgiveness."