﴿ 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!”

Please check ayah 33:17 for complete tafsir.