When you received it with your tongues1 and said with your mouths that of which you had no knowledge and thought it was insignificant while it was, in the sight of Allāh, tremendous.
Translation: Saheeh International (English)
1 Rather than your ears, i.e., not thinking about what you had heard but hastening to repeat it carelessly.
When you were relating it from one another and passing it to one another through your mouths, despite its falsehood, you had no knowledge. You were thinking that is easy and light, whereas it is grave in the sight of Allah, due to it containing lying and accusing an innocent person.