It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that when Pharaoh was drowning, Angel Jibreel (peace be upon him) began to stuff mud into his mouth out of fear that he might say «لَاإِلٰهَإِلَّااللّٰهُ» (and thus escape the punishment for his evil deeds).
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 3154
Hadith Gradingحکم دارالسلام:صحيح موقوفا على ابن عباس