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 Allāh, tremendous.
(Had it not been for the grace of Allah and His mercy unto you in this world and in the Hereafter,) This is addressed to those who were indulging in discussing the matter of `A'ishah, informing them that Allah has accepted their repentance in this world, and forgiven them because of their faith in the Hereafter.
لَمَسَّكُمْ فِى مَآ أَفَضْتُمْ فِيهِ
(would have touched you for that whereof you had spoken.) with regard to the slander.
عَذَابٌ عظِيمٌ
(a great torment) This refers to those who had faith in Allah because of their repentance, such as Mistah, Hassan and Hamnah bint Jahsh the sister of Zaynab bint Jahsh. As for the hypocrites who indulged in the slander, such as `Abdullah bin Ubayy bin Salul and his like, they are not the ones who are referred to in this Ayah , because they did not have sufficient faith and righteous deeds to balance or cancel out what they had done. By the same token, the threats that were narrated for a specific deed are bound to be carried out, if there is no repentance or sufficient righteous deeds to balance or outweigh it. Then Allah says:
When you were welcoming it with your tongues - 24:15.
The word تَلَقَّی means to make inquiries from each other (to be inquisitive) and repeat before others. Here it means to repeat hearsay without probe and proof.(1)
And were taking it as a trivial matter, while in the sight of Allah it was grave - 24:15.
(1). Lexically the word also means 'welcoming'. This meaning is adopted in the translation of the text, because it also covers the meaning of receiving and conveying the news without hesitation. (Editor)
It means that you regarded it a trifle matter that you heard it and passed it on to others. But in the sight of Allah it was a great sin that without probing and finding out any proof you passed on something to
others to cause harm to other Muslims and to make their life miserable.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 13–15
Every right-minded believer must think well of his fellow believer. Entertaining bad opinions about others is actually exposing one’s own evil nature and, conversely, holding a good opinion about others is a proof of one’s own righteous nature. The correct procedure is that whenever anyone speaks ill of any other person, proof should immediately be demanded of him. One who hears such an allegation should not simply start repeating it to others. On the contrary, he should ask the informer to furnish evidence according to the Islamic law to prove that he is right. What he has to say may be worth considering only if he produces witnesses, otherwise he himself becomes the guilty person. Nobody has the right to hold anyone guilty without proof.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.