It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Jibril (peace be upon him) began stuffing mud into Pharaoh's mouth, fearing that he might say 'There is no god but Allah' (and Allah's mercy would reach him)." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), and they did not record it, because most of Shu'bah's students reported it as a saying of Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) (mawquf).