وَيَقُولُ الَّذِينَ كَفَرُوا لَسْتَ مُرْسَلًا ۚ قُلْ كَفَىٰ بِاللَّهِ شَهِيدًا بَيْنِي وَبَيْنَكُمْ وَمَنْ عِنْدَهُ عِلْمُ الْكِتَابِ
And those who disbelieve say: You are not a messenger. Say: Allah is sufficient as a witness between me and you and whoever has knowledge of the Book. (13:43) -Shakir
Excerpt from Tafseer of Agha Mahdi Pooya1:
To every nation one or more messengers of Allah came as warners, and after the last messenger of Allah, there is a divinely appointed guide in every age to administer the affairs of the human society, as a true successor of the prophet.
Imam Ali ibn abi Talib is "he who has the knowledge of the book", and is therefore, along with Allah, a witness of the prophethood of the Holy Prophet.