Musnad Ahmad 25:306

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Adi ibn Thabit and ‘Ata’ ibn al-Sa’ib, from Sa‘id ibn Jubayr, from Ibn ‘Abbas—one of them attributed it to the Prophet (peace and blessings of Allah be upon him)—who said: "Indeed, Gabriel used to stuff mud into the mouth of Pharaoh, for fear that he would say: 'There is no god but Allah.'"