And they had already promised Allah before not to turn their backs and flee. And ever is the promise to Allah [that about which one will be] questioned.
English Transliteration
Walaqad kanoo AAahadoo Allaha min qablu la yuwalloona aladbara wakana AAahdu Allahi masoolan
Tafsir of Verse 15
Yet they had made covenant with God before that, that they would not turn their backs; and covenants with God shall be questioned of
And yet they had already covenanted with Allah not to turn their backs, and a covenant with Allah must (surely) be answered for.