To what extent is the story true that Gabriel (peace be upon him) stuffed earth into Pharaoh's mouth for fear that God's mercy would encompass him while he was drowning?

When Allah drowned Pharaoh, he said, "I believe that there is no god except the one in whom the Children of Israel believe." Gabriel then said, "O Muhammad, if you had seen me, I was taking mud from the sea and stuffing it into his mouth, fearing that mercy might reach him." And in another narration: "Gabriel was stuffing mud into Pharaoh's mouth, fearing that he might say: 'There is no god but Allah.'"