Rashi — Isaiah 6:6

“a glowing coal Heb. רִצְפָּה, a coal, and similarly, (I Kings 19:6) “a cake baked on hot coals (עֻגַּת רְצָפִים),” like עֻגַּת רְשָׁפִים. In regards to Isaiah and Elijah, however, it is written with a ‘zadi,’ רִצְפָּה, because they spoke ill of Israel. This one [Isaiah] called them a people of unclean lips, and this one [Elijah] said, (ibid. 10) “For...have forsaken Your covenant.” Said the Holy One, blessed be He, to the angel, “Break the mouth (רְצוֹץ פֶּה) that spoke ill of My children.” with tongs Heb. (בְּמֶלְקֳחַיִם) with tongs. he had taken it from upon the altar that was in the forecourt.”