When you received it with your tongues and said with your mouths (24:15)

﴿إِذْ تَلَقَّوْنَهُ بِأَلْسِنَتِكُمْ وَتَقُولُونَ بِأَفْوَاهِكُم مَّا لَيْسَ لَكُم بِهِ عِلْمٌ وَتَحْسَبُونَهُ هَيِّنًا وَهُوَ عِندَ اللَّهِ عَظِيمٌ﴾
[ النور: 15]

24:15 When you received it with your tongues and said with your mouths that of which you had no knowledge and thought it was insignificant while it was, in the sight of Allah, tremendous.

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.

When you were propagating it with your tongues, and uttering with your mouths that whereof you had no knowledge, you counted it a little thing, while with Allah it was very great.

phonetic Transliteration

Ith talaqqawnahu bialsinatikum wataqooloona biafwahikum ma laysa lakum bihi AAilmun watahsaboonahu hayyinan wahuwa AAinda Allahi AAatheemun

Abdullah Yusuf Ali - Translation

Behold, ye received it on your tongues, and said out of your mouths things of which ye had no knowledge; and ye thought it to be a light matter, while it was most serious in the sight of Allah.

Safi-ur-Rahman al-Mubarakpuri

When you were propagating it with your tongues, and uttering with your mouths that whereof you had no knowledge, you counted it a little thing, while with Allah it was very great.

Page 351 English transliteration