In the aforementioned verse we read: “Is he who stands on a manifest proof from his Lord, and whom a witness of his own [family] follows? And before him there was the Book of Moses, a guide and mercy.
In the aforementioned verse we read: “Is he who stands on a manifest proof from his Lord, and whom a witness of his own [family] follows? And before him there was the Book of Moses, a guide and mercy. It is they who have faith in it, and whoever defies him from among the factions, the Fire is their tryst. So do not be in doubt about it; it is the truth from your Lord, but most people do not have faith.” Some commentators have said that the term witness is referring to the Angel Gabriel who is the messenger of revelation from God. Others have said that this is referring to the Prophet (ṣ). A third group have interpreted this to be in reference to the tongue of the Prophet (ṣ) and they have understood the term «یَتْلُوهُ» to mean recitation and not as ‘one who follows’. Yet, a large number of commentators have mentioned that this term is in reference to Imam ‘Ali (‘a).
In numerous traditions from the Infallibles (‘a) (some of which have been mentioned in the books of commentary of the Ahl al-Sunnah), this meaning has been emphasized and the ‘witness’ has been referred to as being Imam ‘Ali (‘a). Imam ‘Ali (‘a) is mentioned here as being the first man to believe in the Prophet (ṣ) and the Quran; up until the very end, he was always by the side of the Prophet (ṣ) and did everything possible to help and support him.
In a tradition, Imam ‘Ali (‘a) is narrated to have said: There are singular or multiple verses revealed in regards to all the famous men of the Quraysh. Someone asked: Oh Amīr al-Mu’minīn, which verse was revealed in regards to you? The Imam (‘a) said: Have you not read the verse in Surah Hūd which says: “Is he who stands on a manifest proof from his Lord, and whom a witness of his own [family] follows?” The Prophet (ṣ) is the one who stands on a manifest proof and I am the witness from his own family.
In the last verse of Surah Ra’ad, there is a term which further emphasizes this meaning: “The faithless say, ‘You have not been sent [by Allah].’ Say, ‘Allah suffices as a witness between me and you, and he who possesses the knowledge of the Book.’” In many traditions from both Shia and Sunni chains of narration, we see traditions which mention the meaning of the words “…and he who possesses the knowledge of the Book.” as being in reference to Imam ‘Ali (‘a). In a tradition from Imam Baqir (‘a), it has been narrated that: The meaning of the term witness is Imam ‘Ali (‘a), and it then refers to his successors, who come one after the other. It can also be understood that this term does not only encompass one individual, it is rather referring to all individuals who are worthy of this rank.
 «أَ فَمَنْ کانَ عَلى بَیِّنَة مِنْ رَبِّهِ وَ یَتْلُوهُ شاهِدٌ مِنْهُ وَ مِنْ قَبْلِهِ کِتابُ مُوسى إِماماً وَ رَحْمَةً»  Tafsīr burhān, vol. 2, p. 211; Nūr al-thaqalayn, Qurṭubī, vol. 9, p. 16; Majma’ al-bayān…  Ibid.  Ibid.  Ibid.  Tafsīr burhān, vol. 2, p. 211; Nūr al-thaqalayn, Qurṭubī, vol. 9, p. 16; Majma’ al-bayān…  Surah Ra’ad, verse 43: «وَ یَقُولُ الَّذینَ کَفَرُوا لَسْتَ مُرْسَلاً قُلْ کَفى بِاللّهِ شَهیداً بَیْنی وَ بَیْنَکُمْ وَ مَنْ عِنْدَهُ عِلْمُ الْکِتابِ»  Shia sources: Al-Kāfī, vol. 1, p. 229, hadith 6 (Dār al-Kutub al-Islāmīyah); Wasā’il al-shī’ah, vol. 27, p. 181, hadith 33546 (Āl al-Bayt Edition), Biḥār al-anwār…  Tafsīr burhān, vol. 2, p. 211, hadith 8 (vol. 3, p. 212, hadith 8, Bi’thah Print); Biḥār al-anwār, vol. 35, p. 388, hadith 6; Tafsīr ‘Ayāshī, vol. 2…  Taken from the text: Tafsīr nemūneh, Ayatullah Makarem Shirazi, Dār al-Kutub al-Islāmīyah, 25th Edition, vol. 9, p. 73.