Ma'arif al-Qur'an
Mufti Muhammad Shafiوَلَوْلَا إِذْ سَمِعْتُمُوهُ قُلْتُم مَّا يَكُونُ لَنَا أَن نَّتَكَلَّمَ بِهَـٰذَا سُبْحَانَكَ هَـٰذَا بُهْتَانٌ عَظِيمٌAnd why, when you heard of it, did you not say, It is not for us to speak about this. Pure are You (0 Allah). This is a terrible calumny.' - 24:16.
In this verse the same advice is repeated which was imparted in an earlier verse, with further elaboration as to what should the Muslims do in such a situation when they hear a rumor. The advice is that they should say in categorical terms that they are not allowed even to mention such a thing for which there is no proof. It is a very grave accusation.
A doubt and its answer
If someone has a doubt that just as the truth of an event cannot be verified without a proof, and hence it is unlawful to mention it and spread it, likewise, the report of that event should not be believed to be a lie and a 'terrible calumny' without a proof of its being a lie. The answer to this doubt is that it is the integral spirit of Islam to consider every Muslim as free from sins, and this is based on reasoning. Anything contrary to this proven reality is a lie if put forward without a proof, and no further proof is required to prove this assertion a lie. This is enough to call it a slander if a believer Muslim is blamed for a crime without proof.