Musannaf Ibn Abi Shayba 21:251

Waki’ narrated to us, from Zakariya ibn Ishaq, he said: Yahya ibn Abdullah ibn Sayfi narrated to me, from Abu Ma’bad, from Ibn Abbas, from Mu’adh ibn Jabal, from the Prophet, peace and blessings of Allah be upon him, who said: "Beware of the supplication of the oppressed, for there is no veil between it and Allah."