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Narrated Abu Bakr: I said to the Prophet (ﷺ) 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?"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ سِنَانٍ، حَدَّثَنَا هَمَّامٌ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، عَنْ أَبِي بَكْرٍ ـ رضى الله عنه ـ قَالَ قُلْتُ لِلنَّبِيِّ صلى الله عليه وسلم وَأَنَا فِي الْغَارِ لَوْ أَنَّ أَحَدَهُمْ نَظَرَ تَحْتَ قَدَمَيْهِ لأَبْصَرَنَا. فَقَالَ " مَا ظَنُّكَ يَا أَبَا بَكْرٍ بِاثْنَيْنِ اللَّهُ ثَالِثُهُمَا ".
sunnah.com