This is further discipline, in addition to the command to think well of people, i.e., if something unbefitting is mentioned about good people, then one should think well of them, and not feel towards them anything but good. Then if a person has any unsuitable thoughts about them, insinuated into his mind and imagination by Shaytan, he should not speak about that, for the Prophet ﷺ said:

(Allah will excuse my Ummah for anything that occurs to their minds, so long as they do not speak about it or act upon it.) This was reported in the Two Sahihs. Allah's saying:

(And why did you not, when you heard it, say: "It is not right for us to speak of this".) meaning, we should not talk about it or mention it to anyone.

(Glory be to You (O Allah)! This is a great lie.) means, glory be to Allah that such a thing should be said about the wife of His Prophet and close Friend ﷺ. Then Allah says,

(Allah forbids you from it and warns you not to repeat the like of it forever,) meaning, Allah is forbidding you and warning you from doing anything like this again in the future. Allah says,

(if you are believers.) meaning, if you believe in Allah and His Laws, and you respect His Messenger ﷺ. As for those who are described as disbelievers, a different ruling applies in their case. Then Allah says,

(And Allah makes the Ayat plain to you,) meaning, He makes clear to you the rulings of Shari'ah and His divine decrees.

(and Allah is All-Knowing, All-Wise.) means, He knows what is right for His servants and He is Wise in His Laws and decrees.