(… أَلَمْ تَرَ ) (Do you not consider,…): It’s such a great directive by the Almighty to His Prophet (saws), that is: if Allah (swt) tells you something, what He tells is more credible than your eyesight.
(… إِلَى الَّذِينَ أُوتُوا نَصِيبًا مِّنَ الْكِتَابِ …) (…those who were given a portion of the Scripture?…): The talk is still about the Jews. But why it’s a portion of the Scripture? Because the portion they have is altered but the other portion is unaltered.