Tafsir of Ar-Ra'd 13:43

{كفى بالله شهيداً} By virtue of the proofs He has manifested regarding my messengership.

{ومن عنده علم الكتاب} He who possesses the knowledge of the Qur’an and the miraculous composition upon which it is structured, which exceeds human capacity.

It is also said: It refers to those among the scholars of the People of the Book who have embraced Islam, for they bear witness to his [the Prophet's] description in their scriptures.

It is also said: It is Allah, Mighty and Majestic is He, and "the Book" is the Preserved Tablet (al-Lawḥ al-Maḥfūẓ). Al-Hasan said: "No, by Allah, it means none other than Allah." The meaning is: "Sufficient as a witness between me and you is He who is worthy of worship, and He who alone knows what is in the Preserved Tablet."

This is supported by the reading: (Wa-man ‘indahu ‘ilmu al-kitābi), where ‘ilmu is raised by the prepositional phrase (al-ẓarf), acting as a subject (fā‘il). This is because when a prepositional phrase functions as a relative clause (ṣilah), it becomes deeply assimilated to the verb due to its reliance on the relative pronoun, thus acting like a verb—similar to saying: "I passed by the one in whose house is his brother" (marartu bi-alladhī fī al-dāri akhūhu), where "his brother" is the subject, just as if you said: "the one whose brother has settled in the house." In the reading where ‘indahu is not a relative clause, ‘ilmu is raised by way of ibtidā’ (as an initial subject).

From the Messenger of Allah (ﷺ): "Whoever recites Surah Ar-Raʿd will be given a reward of ten good deeds for the weight of every cloud that has passed and every cloud that will be until the Day of Resurrection, and he will be resurrected on the Day of Resurrection among those who fulfill the covenant of Allah."