Anas b. Malik reported that Abu Bakr Siddiq reported him thus: I saw the feet of the polytheists very close to us as we were in the cave. I said: Allah's Messenger, if one amongst them were to see at his feet he would have surely seen us. Thereupon he said: Abu Bakr, what can befall two who have Allah as the third One with them.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، وَعَبْدُ بْنُ حُمَيْدٍ، وَعَبْدُ اللَّهِ بْنُ عَبْدِ الرَّحْمَنِ الدَّارِمِيُّ، قَالَ عَبْدُ اللَّهِ أَخْبَرَنَا وَقَالَ الآخَرَانِ، حَدَّثَنَا حَبَّانُ بْنُ هِلاَلٍ، حَدَّثَنَا هَمَّامٌ، حَدَّثَنَا ثَابِتٌ، حَدَّثَنَا أَنَسُ بْنُ مَالِكٍ، أَنَّ أَبَا بَكْرٍ الصِّدِّيقَ، حَدَّثَهُ قَالَ نَظَرْتُ إِلَى أَقْدَامِ الْمُشْرِكِينَ عَلَى رُءُوسِنَا وَنَحْنُ فِي الْغَارِ فَقُلْتُ يَا رَسُولَ اللَّهِ لَوْ أَنَّ أَحَدَهُمْ نَظَرَ إِلَى قَدَمَيْهِ أَبْصَرَنَا تَحْتَ قَدَمَيْهِ فَقَالَ " يَا أَبَا بَكْرٍ مَا ظَنُّكَ بِاثْنَيْنِ اللَّهُ ثَالِثُهُمَا " .
sunnah.com