´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."
Hadith Referenceسنن ترمذي / كتاب تفسير القرآن عن رسول الله صلى الله عليه وسلم / 3108
´Narrated Ibn 'Abbas:` that the Messenger of Allah (ﷺ) said: "When Allah drowned Fir'awn he said: 'I believe that there is no god except the One that the children of Isra'il believe in.' So Jibrail said: 'O Muhammad! If you could only have seen me, while I was taking (the mud) from the sea, and filling his mouth out of fear that the mercy would reach him.'"
Shaykh Dr. Abdur Rahman Freywai
Commentary: Note: (In the chain of narration, Ali bin Zayd bin Jud'an is a weak narrator. However, by being supported by the hadith that follows below, this hadith is authentic.)
Source: Sunan al-Tirmidhi – Majlis ‘Ilmi Dar al-Da‘wah, New Delhi Edition, Page: 3107