What is the ruling on the muezzin interrupting the Witr prayer at the bowing (ruku') for the Fajr Adhan, then completing it from the point of interruption after the Adhan?

This person's action is impermissible, and his prayer is invalidated by him cutting it off. He must resume it. He could have completed the Witr prayer with a light rak'ah and then given the adhan, because a slight delay does not harm. His prayer is invalidated whether he intended to cut it off or not, because the adhan contains human speech, which is represented by the muezzin's call, "Hurry to the prayer."