Appears 4 times in the Quran.
9:40 إِلَّا تَنصُرُوهُ فَقَدْ نَصَرَهُ ٱللَّهُ إِذْ أَخْرَجَهُ ٱلَّذِينَ كَفَرُوا۟ ثَانِىَ ٱثْنَيْنِ إِذْ هُمَا فِى ٱلْغَارِ إِذْ يَقُولُ لِصَٰحِبِهِۦ لَا تَحْزَنْ إِنَّ ٱللَّهَ مَعَنَا فَأَنزَلَ ٱللَّهُ سَكِينَتَهُۥ عَلَيْهِ وَأَيَّدَهُۥ بِجُنُودٍ لَّمْ تَرَوْهَا وَجَعَلَ كَلِمَةَ ٱلَّذِينَ كَفَرُوا۟ ٱلسُّفْلَىٰ وَكَلِمَةُ ٱللَّهِ هِىَ ٱلْعُلْيَا وَٱللَّهُ عَزِيزٌ حَكِيمٌ
If you will not aid him, Allah certainly aided him when those who disbelieved expelled him, he being the second of the two, when they were both in the cave, when he said to his companion: Grieve not, surely Allah is with us. So Allah sent down His tranquillity upon him and strengthened him with hosts which you did not see, and made lowest the word of those who disbelieved; and the word of Allah, that is the highest; and Allah is Mighty, Wise.
9:57 لَوْ يَجِدُونَ مَلْجَـًٔا أَوْ مَغَٰرَٰتٍ أَوْ مُدَّخَلًا لَّوَلَّوْا۟ إِلَيْهِ وَهُمْ يَجْمَحُونَ
If they could find a refuge or cave or a place to enter into, they would certainly have turned thereto, running away in all haste.
Or its waters should sink down into the ground so that you are unable to find it.
Say: Have you considered if your water should go down, who is it then that will bring you flowing water?