﴿ وَلَقَدْ كَانُوا۟ عَٰهَدُوا۟ ٱللَّهَ مِن قَبْلُ لَا يُوَلُّونَ ٱلْأَدْبَٰرَ ۚ وَكَانَ عَهْدُ ٱللَّهِ مَسْـُٔولًۭا ﴾

“although ere that they had vowed before God that they would never turn their backs [on His message]: and a vow made to God must surely be answered for!”

(And verily they had already sworn unto Allah) before the battle of the Ditch, on the occasion of the battle of the Clans (that they would not turn their backs) fleeing from the idolaters. (An oath to Allah) the one who breaks his oath to Allah (must he answered for) will be asked about it on the Day of Judgement.