Narrated Abu Bakr:

I said to the Prophet while I was in the Cave. "If any of them should look under his feet, he would see us." He said, "O Abu Bakr! What do you think of two (persons) the third of whom is Allah?"

حَدَّثَنَا مُحَمَّدُ بْنُ سِنَانٍ، حَدَّثَنَا هَمَّامٌ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، عَنْ أَبِي بَكْرٍ ـ رضى الله عنه ـ قَالَ قُلْتُ لِلنَّبِيِّ ﷺ وَأَنَا فِي الْغَارِ لَوْ أَنَّ أَحَدَهُمْ نَظَرَ تَحْتَ قَدَمَيْهِ لأَبْصَرَنَا. فَقَالَ
" مَا ظَنُّكَ يَا أَبَا بَكْرٍ بِاثْنَيْنِ اللَّهُ ثَالِثُهُمَا ".