In the hours before dawn “they were (found) asking (Allah) for forgiveness”
As-Sa‘di said:“And in the hours before dawn”, which is just before dawn “they were (found) asking (Allah) for forgiveness”.
So they made their prayer last until the time just before dawn, then they sat, concluding their night-time worship (qiyaam al-layl) by asking Allah, may He be exalted, for forgiveness, in the manner of a sinner seeking forgiveness for his sins.
There is a special virtue in praying for forgiveness in the time before dawn that is not present at any other time, as Allah, may He be exalted, says describing the people of faith and obedience (interpretation of the meaning):
“and those who pray and beg Allah’s Pardon in the last hours of the night” [Aal ‘Imraan 3:17].
— Tafseer as-Sa‘di p. 809 – تفسير السعدي