Tafsir of Al-Qasas 28:35

"We will strengthen your arm" The ‘aḍud (upper arm) is the support of the hand; through its strength, the hand becomes strong. Ṭarafa said: O sons of Labīnī, you are not a hand, Except a hand that has no upper arm.

It is said in a prayer for good: "May Allah strengthen your arm (shadda Allāhu ‘aḍudaka)," and in its opposite: "May He weaken your arm (fatta Allāhu fī ‘aḍudika)."

The meaning of "We will strengthen your arm with your brother" is: We will empower you through him and assist you. This is either because the hand gains strength through the strength of the upper arm—and the body gains strength through the hand’s ability to manage affairs—or because a man is likened to a hand that gains strength through the strengthening of the upper arm; thus, he is made as if he were a hand strengthened by a powerful arm.

"Authority" (sulṭānan) Dominance and power, or a clear proof.

"By Our signs" (bi-āyātinā) This relates to something similar to its relation in "with nine signs" (i.e., "Go both of you with Our signs"). Or, it relates to "We will grant you both authority," meaning: We will empower you both through Our signs. Or, it relates to "They shall not reach you," meaning: You will be protected from them by Our signs.

Alternatively, it is an explanation for "the victors" (al-ghālibūn), not a ṣilah (relative clause), because a ṣilah cannot precede the mawṣūl (relative pronoun). If it were placed after, it would necessarily be a ṣilah for it. It is also possible that it is an oath whose response is "they shall not reach you," placed before it, or it is a redundant oath (laghw al-qasam).

"But when Moses came to them with Our signs as clear proofs, they said, 'This is nothing but invented magic, and we never heard of this among our forefathers of old.'"