Tafsīr al-Qummi: He said, it was narrated to me by my father, from Ḥammād:

From Abī ʿAbd Allāh عليه السلام, regarding His saying: the straight path, he said: That is ‘Amīr al-Mu’minīn عليه  السلام, and recognition of him – and the proof that it is ‘Amīr al-Mu’minīn is His saying: ‘And indeed it is, in the Mother of the Book with Us, is ʿAlī, a wise man.’1 2 and it is ‘Amīr al-Mu’minīn in the Mother of the Book.

1. Al-Zukhruf (43:4)
2. Most translations suggest it is: “And indeed it is, in the Mother of the Book with Us, exalted and full of wisdom.” However, according to this ḥadīth, the Imām suggests it means it is ʿAlī. The Arabic would not change, as the Arabic word could indeed mean both things. In regards to this, al-Majlisi says: “In many traditions from Ibn Mahyar and others, Imām al-Ṣādiq and Imam Ridha (as) are reported to have said, while explaining the verse that it implies that the Qur’ān in the protected tablet is with Us. It is protected from any changes or alterations and it is the highest in rank among all Divine scriptures and it is full of wisdom, that is, it contains many words of wisdom or it is Moḥkam (clear) and it is not cancelled or amended but by itself. These gentleman have said that it means that ʿAlī (as) is mentioned in Surah al-Fātiḥa which is Umm al-Kitāb (Mother of the Book) who is both wise and intelligent. And it depends upon the fact that Imam ʿAlī (as) is the straight path in the path of guardianship and following. Hence it is said that when people asked Imam Ridha (as) as to where Imam ʿAlī bin Abī Ṭālib has been mentioned in Umm al-Kitab. He replied: in ―”Guide us to the straight path” because ʿAlī (as) is the Ṣīrāt Mustaqīm and it is mentioned in the prayer of the day of Ghadīr that: I give witness that ʿAlī (as) is the guiding Imām and that he is the Master of the Believers whose mention has been made by Allāh in His book, saying: And surely it is in the mother of the Book with Us, (ʿAlī ) truly elevated, full of wisdom.” [Ḥayāt al-Qulūb, al-Majlisi, English version translated by Syed Athar Husain S.H.Rizvi, Al-Qalam Translators & Writers Bureau, Published by Ansariyan, Volume 3, Pages #145-146]

قال : وحدثني أبي ، عن حماد ، عن أبي عبدالله ( عليه السلام ) في قوله : * ( الصِّراطَ الْمُسْتَقِيمَ ) * . قال : « هو أمير المؤمنين ( عليه السلام ) ومعرفته ، والدليل على أنه أمير المؤمنين قوله : وإِنَّه فِي أُمِّ الْكِتابِ لَدَيْنا لَعَلِيٌّ حَكِيمٌ ) * وهو أمير المؤمنين ( عليه السلام ) في أم الكتاب . »