أَفَمَنْ كَانَ عَلَىٰ بَيِّنَةٍ مِنْ رَبِّهِ وَيَتْلُوهُ شَاهِدٌ مِنْهُ وَمِنْ قَبْلِهِ كِتَابُ مُوسَىٰ إِمَامًا وَرَحْمَةً ۚ أُولَٰئِكَ يُؤْمِنُونَ بِهِ ۚ وَمَنْ يَكْفُرْ بِهِ مِنَ الْأَحْزَابِ فَالنَّارُ مَوْعِدُهُ ۚ فَلَا تَكُ فِي مِرْيَةٍ مِنْهُ ۚ إِنَّهُ الْحَقُّ مِنْ رَبِّكَ وَلَٰكِنَّ أَكْثَرَ النَّاسِ لَا يُؤْمِنُونَ
Is he then who has with him clear proof from his Lord, and a witness from Him recites it and before it (is) the Book of Musa, a guide and a mercy? These believe in it; and whoever of the (different) parties disbelieves in it, the fire will be their promised meeting-place. Be not then in doubt thereon: surely it is the truth from your Lord, but most men do not believe. (11:17) -Shakir
Excerpt from Tafseer of Agha Mahdi Pooya1:
There are several traditions reported on the authority of Ahl ul-Bayt, and also narrated by well-known Muslim scholars (see Appendix H) that shahid in this verse refers to Ali ibn abi Talib, just as shahid in verse 43 of Ar-Ra’d (13:43) also refers to Imam Ali.
In reply to a question Imam Ali said that in "Is he then (like unto him) who has a clear proof from his Lord, and a witness, from Him, follows him," the Holy Prophet is the divine "bayyanah" and I am the witness, guide and mercy.
Allah has declared Ali to be the Imam who alone testifies the truth about Allah and His Prophet, and like the Holy Prophet who is "mercy unto the world", he is also "mercy' because both of them are from one and the same divine light, therefore, Ali is the only true successor of the Holy Prophet whom all the Muslims should follow if they have truly and sincerely surrendered themselves to the will of Allah.