Quran 33:15 from Surah Al-Ahzaab on Divine Marvels.
وَلَقَدۡ كَانُواْ عَٰهَدُواْ ٱللَّهَ مِن قَبۡلُ لَا يُوَلُّونَ ٱلۡأَدۡبَٰرَۚ وَكَانَ عَهۡدُ ٱللَّهِ مَسۡـُٔولٗا
English translation: And certainly they had promised Allah before before not they would turn their backs And is (the) promise (to) Allah to be questioned
Transliteration: walaqad kanu ahadu al-laha min qablu la yuwalluna al-adbara wakana ahdu al-lahi masulan