Musnad Ahmad 25:1295

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, ‘Adi ibn Thabit and ‘Ata’ ibn al-Sa’ib narrated to us, from Sa‘id ibn Jubayr, from Ibn ‘Abbas, who said: One of them attributed it to the Prophet, may God bless him and grant him peace: "Indeed, Gabriel was stuffing mud into the mouth of Pharaoh, fearing that he might say: 'There is no god but God.'"