Tafsir of Ar-Ra'd 13:7

{Why has a sign not been sent down to him from his Lord?} They did not regard the signs sent down upon the Messenger of Allah (ﷺ) due to stubbornness. Thus, they proposed signs like those of Moses and Jesus—such as the staff turning into a serpent or the reviving of the dead.

It was said to the Messenger of Allah (ﷺ): You are merely a man sent as a warner, frightening them of an evil outcome, and a counselor like the other messengers. You are only required to bring that which validates your status as a warning messenger. The validity of this is achieved by any sign whatsoever; all signs are equal in establishing the truth of the claim, with no disparity between them. He, with whom everything is by measure, grants every prophet a sign according to what His knowledge of interests requires and His estimation of them.

{And for every people is a guide} From the prophets, who guides them to the religion and calls them to Allah through a specific mode of guidance and a sign with which he was distinguished. He did not make the prophets a single law regarding specific signs.

Another interpretation: The meaning is that they deny that what has been sent down to you are signs, and they are stubborn. Do not let that concern you; you are only a warner. You are not required to establish faith in their hearts, nor are you capable of it. {And for every people is a guide}—One capable of guiding them by compulsion, and that is Allah, the Exalted.

By following this with the mention of the signs of His knowledge and His estimation of things, He indicated through the decrees of His wisdom that His granting each warner signs different from the signs of others is a matter managed by penetrating knowledge and ordained by divine wisdom. Had He known any good or benefit in answering their proposal, He would have answered it.

As for the second interpretation, it indicates that He who possesses this power and this knowledge is the only One capable of guiding them, the One who knows the path by which to guide them, and there is no way for anyone else to do so.

{Allah knows what every female carries and what the wombs lose and what they exceed. And everything with Him is by measure. Knower of the unseen and the witnessed, the Grand, the Exalted.}