Musannaf Ibn Abi Shayba 21:253

Muhammad ibn Bishr narrated to us, Mis'ar narrated to us, from Ma'n, from 'Awn ibn 'Abd Allah, who said: Four [things] are not veiled from Allah: the supplication of a parent who is pleased, a just leader, the supplication of the oppressed, and a man's supplication for his brother in his absence.