Yeshaya Perek 6
Yeshaya saw a form of Divine vision, who was sitting on a high and lofty throne, and the seraphim were praising Him. Yeshaya says, "Woe is me, for I am doomed, as I am a man of impure lips and I dwell among a people with impure lips." (Yeshaya 6:5) One of the seraphim takes a coal from the altar and places it on Yeshaya's lips, telling him, "Behold, this has touched your lips; your sin has gone away and your sin shall be atoned for." (6:7) What was Yeshaya's sin, and why was coal used?
The Gemara in Yevamos 49b relates that Yeshaya's mouth was impaired because he slandered Bnei Yisrael when he said, "I dwell among a people with impure lips." Rashi writes, "He was punished that his mouth was impaired because he called Bnei Yisrael a people with impure lips of his own accord, not by Hashem’s command or as rebuke."
The Yalkut Shimoni similarly writes (II:406): “When he [Yeshaya] saw the heavenly angels praising Hashem and did not join in their praise, he felt bad, "For I am a man of impure lips," for had I joined my praise with theirs I would live forever like them ... While he was still standing and contemplating this, he uttered an extra phrase, "And I dwell among a people with impure lips." Hashem said to him: Regarding what you said, "I am a man of impure lips," you are pardoned since you are master over yourself. But are you master over my children that you said, "And I dwell among a people with impure lips?" Immediately, he was punished, as it says ... "In his hand was a 'ritzpa' (coal)". "Ritzpa" is used and not "gacheles" (ember) as Rav Shmuel bar Nachmani says: "Ratzetz peh" (Smash the mouth) which spoke lashon hora about my sons.”
Eliyahu also complained against Bnei Yisrael and said, "They have forsaken Your covenant." (Melachim I 19:10) There, too, Hashem prepared for him an "ugat retzafim" (coal-baked cake), which according to Rav Shmuel bar Nachmani, is an allusion to smashing the mouth of one who slanders Bnei Yisrael.
The "ritzpa," the coal, serves as a hint to lashon hora because its damage continues long after the criminal act of speaking is over. The Talmud Yerushalmi (Pe'ah 1:1) explains the idea of the "rotem" tree that Eliyahu slept under. "All other coals, when they are extinguished on the outside they are extinguished inside [as well]; whereas these [coals of rotem], even though they are extinguished on the outside they are not extinguished inside. The story is told of a person who left coals burning on Succos and found them still burning on Pesach." This shows the long lasting effect of lashon hora, and demonstrates the connection between the coal and the sin of Yeshaya.