فَسَمِعْتُهُ يَقُولُ حِينَ خَبَّ فِي مَجَالِهِ
Then I heard him speak as he gathered speed in his field of oratory.
Khabba suggests moving briskly; majāl is a field or arena, here the arena of speech. Together they depict fluent, energetic oratory.
Ḥīna gives the time: the narrator listens just as the speaker’s address is accelerating.