Allah the Almighty States:
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
Can he, then, who possesses a clear proof from his Lord, and to testify to whose truth a witness from Him shall follow him, and who was preceded by the Book of Moses, a guide and a mercy, be an impostor? Those who consider these matters believe therein, and whoever of the opposing parties disbelieves in it, Fire shall be his promised place. So be not thou in doubt about it. Surely, it is the truth from thy Lord; but most men do not believe.
In this verse, Allah speaks of a reformer who would come from the ummah of the Prophet Muhammad (sa) and be his follower. Hadrat Musleh Maud (ra)’s notes beautifully explain:
The words, ‘a witness from Him (God) shall follow him (the Holy Prophet), also point to the appearance of a great Reformer in the latter Days who was to come as “a witness” to bear testimony to the truth of the Holy Prophet (sa). It is worth noting that the person who was to appear after the Holy Prophet to bear testimony to his truth has been called a shaahid (a witness) and it is evident that a witness is needed only when the continued practicability and usefulness of a Message is called in question. It is evident, therefore, that after the advent of the Holy Prophet a Messenger was needed only at a time when there should arise about the Quran the question whether it was still worthy of being acted upon. It is well known that such a question never arose about the Quran during the last 1350 years, and it is only in the present time that doubts have begun to be expressed about this claim of the Quran with persistence from all quarters. There is a section of Muslims themselves who hold the view that some teachings of the Quran, particularly with regard to the five daily Prayers, the amputation of the hands of a thief, polygamy, purdah, usury, etc., need alteration or amendment. Then there are the followers of claimants like Baha’ullah, who believe that the Law of Islam has become abrogated and they seek to introduce a new Law in its place. Last of all, the so-called higher critics of the west question the authenticity of some historical facts mentioned in the Quran as well as the practicability of some of its teachings. Such a state of affairs never existed before the present age and therefore, truly speaking, no “witness” was needed before now.
The words منه) from Him) in the expression, a witness from Him, clearly show that the promised witness was to be a heavenly Messenger. This heavenly Messenger is Ahmad, the Promised Messiah, Founder of the Ahmadiyya Movement, who appeared at a time when the teachings of Islam had begun openly to be declared unpractical and impracticable and the object of whose advent was to bear witness to the truth of Islam by means of clear
proofs and fresh heavenly signs.
It should also be remembered that the words, a witness from Him shall follow him, promised the appearance
of a person who was to invite men to accept and follow the Quran and not of one who was to abrogate its teachings. Thus, incidentally, the verse furnishes an irrefutable argument against the claims of Baha’ullah.