تَوَقَّفُوا لَيْلًا، ثُمَّ سَارُوا صَبَاحًا.
They stopped for the night and then traveled in the morning.
Thumma expresses order with an interval. In this narrative it fits a night's halt followed by departure the next morning.
Fa-sārū would normally suggest closer succession; wa-sārū would merely join the propositions. The particles must be read together with temporal expressions such as laylan and ṣabāḥan; otherwise the implied timeline changes.