إِلَّا تَنصُرُوهُ فَقَدْ نَصَرَهُ اللَّهُ إِذْ أَخْرَجَهُ الَّذِينَ كَفَرُوا ثَانِيَ اثْنَيْنِ إِذْ هُمَا فِي الْغَارِ إِذْ يَقُولُ لِصَاحِبِهِ لَا تَحْزَنْ إِنَّ اللَّهَ مَعَنَا ۖ فَأَنزَلَ اللَّهُ سَكِينَتَهُ عَلَيْهِ وَأَيَّدَهُ بِجُنُودٍ لَّمْ تَرَوْهَا وَجَعَلَ كَلِمَةَ الَّذِينَ كَفَرُوا السُّفْلَىٰ ۗ وَكَلِمَةُ اللَّهِ هِيَ الْعُلْيَا ۗ وَاللَّهُ عَزِيزٌ حَكِيمٌ
“[It does not matter] if you [Believers] do not support him (the Prophet ﷺ), for Allah did in fact support him when the disbelievers drove him out [of Makkah], and he was only one of two. While they both were in the cave, he reassured his companion, "Do not worry; Allah is certainly with us." So Allah sent down His serenity upon the Prophet, supported him with forces you did not see, and made the word of the disbelievers lowest, while the Word of Allah supreme. And Allah is Almighty, All-Wise.”
Revealed about the Prophet ﷺ and Abu Bakr RA in the Cave of Thawr; the only Qur'anic verse to directly affirm a specific sahabi's companionship.
This ayah recalls Abu Bakr RA and the Prophet ﷺ's reassurance in the Cave of Thawr during the Hijrah -- the journey that began on this very night, when Ali RA stayed behind as a decoy.