For ALLAH is well aware of all that ye do.

Bismillah Ar Rahman Ar Raheem
"Behold ye were climbing up the high ground,
without even casting a side glance at anyone,and
the Messenger in your rear was calling you back.
There did ALLAH give you one distress after
another by way of requital,to teach you not to
grieve for (the booty) that had escaped you and for
(the ill) that had be fallen you. For ALLAH is well
aware of all that you do. "
[Surah Ali Imran 3:153]
It would seem that a party of horsemen led by the
dashing Khalid ibn Walid came through the gap in
the passes where the Muslim archers should have
been,and in the confusion that arose,the retreating
foe rallied and turned back on the Muslim. From
the low ground on the bank of the Nullah,the
Muslims retreated in their turn and tried to gain the
hill. They had a double loss: (1) They were battled
of the booty they had run after,and (2) their own
lives and the lives of their whole army were in
danger,and many lives were actually lost from their
ranks. Their own lives being in danger; they had
hardly time grieve for the loss of booty or the
general calamity,but it steadied them,and some of
them stood the test.
Subhana ALLAH