[They will not hear therein any ill speech (la yasma'una fiha laghwan)]": al-laghw is falsehood. "[Nor any falsehood (wa la kithaban)]": al-Hasan's explanation is that they do not call one another liars.
Muhammad said: Whoever recites it as (kadhdhaban) with a shadda (doubled consonant), it is from their saying: "kadhdhab" (liar) and "kadhdhaba" (to lie), meaning the same thing.