وَلَقَدْ رَءَاهُ بِٱلْأُفُقِ ٱلْمُبِينِ
The report, that is, did not arrive anonymously. The link between is not an indirect intuition or an inner voice; it is an encounter that was seen.
With this the argument from a chain of transmission is completed: the source (the Owner of the Throne) → the carrier (the messenger whose qualities were enumerated) → a delivery that was seen → the receiver (your companion).
The grammar. Wa laqad — wāw + lām + qad: the lām of the oath and the particle of verification. It conveys a threefold certainty.
The view of the majority: the Prophet saw Gabriel. Its ground is directly the context — the three preceding verses describe the angel, the following verse speaks of the Prophet; the pronouns are distributed between these two.
"He was taught it by one mighty in powers. Endowed with sound judgement; he stood erect — and he was on the highest horizon (وَهُوَ بِٱلْأُفُقِ ٱلْأَعْلَىٰ). Then he drew near and came down." (al-Najm 53/5-8)
The two sūras are looking at the same event. al-Takwīr says al-ufuqi'l-mubīn (the clear horizon), al-Najm al-ufuqi'l-aʿlā (the highest horizon).
A qualification. Whom the event of seeing in Sūrat al-Najm points to — Gabriel, or God — has been disputed in the history of exegesis, and differing views are transmitted among the Companions as well. That dispute belongs to Sūrat al-Najm and should be treated there. In al-Takwīr the dispute is narrower, because the three verses immediately before it plainly describe a messenger.
Root: أ-ف-ق. The horizon, the line where sky and earth meet; āfāq — its plural, the quarters of the sky, far countries. In the Qurʾān: "We shall show them Our signs on the horizons and in themselves" (Fuṣṣilat 41/53: fi'l-āfāqi wa fī anfusihim).
Root: ب-ي-ن. This root was treated in detail in the section on al-Bayyina 98/1; I do not repeat it here. In summary: beneath the root lie separating, opening a space between — bayn (the space between), bayān (explanation, distinguishing), bayyina (the clear proof that distinguishes), mubīn (clear, distinguishing).
And the word's placing here is fine: the horizon is the line at which sky and earth are separated. The root's sense of "separating" and the horizon itself carry the same idea.
And its function: mubīn says that the seeing took place in conditions that leave no room for doubt. This is the first question to be asked of an eyewitness: how did you see, what was the weather, what was the distance. The verse answers that question in advance.