Translation:

153. Behold! ye were climbing up the high ground without even casting a side glance at anyone and the Apostle 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 befallen you. For Allah is well aware of all that ye do.

Notes (Tafseer)

464. It would seem that a party of horsemen led by the dashing Khalid ibn Al-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 Muslims. From the low ground on the bank of the valley the Muslims retreated in their turn and tried to gain the hill. They had a double loss: (1) they were baulked 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 to grieve for the general calamity. But it steadied them, and some of them stood the test.