{ Every messenger and prophet that We have sent forth before you, when they recited, Satan cast something into what they intended; but Allah abrogated what Satan cast; then Allah affirms his verses. And Allah is knowing, wise. } Al-hajj 52

The reason why this verse was sent down to the Messenger of Allah (SAW) was that Satan had put things on his tongue in some of what He recited of the Qur’an that Allah had sent down to him – things that Allah did not send down to him. And this got intense for the Messenger of Allah (SAW) and He became distressed over it, and so Allah comforted him of it with these verses.