Narrated Shu'bah:
"'Adi bin Thabit and 'Ata bin As-Sa'ib informed me, from Sa'eed bin Jubair, from Ibn 'Abbas - and one of them mentioned that it was from the Prophet (ﷺ) - that he mentioned that Jibra'il began shoving clay in the mouth of Fir'awn out of fear that he would say La Ilaha Illallah and Allah would have mercy upon him - or fearing that Allah would have mercy upon him." (JA)
English reference (JA): Vol. 5, Book 44, Hadith 3108 (JA)
Arabic reference (JA): Book 47, Hadith 3393 (JA)