The Quran

أَلَم تَرَ أَنَّ اللَّهَ يَعلَمُ ما فِي السَّماواتِ وَما فِي الأَرضِ ۖ ما يَكونُ مِن نَجوىٰ ثَلاثَةٍ إِلّا هُوَ رابِعُهُم وَلا خَمسَةٍ إِلّا هُوَ سادِسُهُم وَلا أَدنىٰ مِن ذٰلِكَ وَلا أَكثَرَ إِلّا هُوَ مَعَهُم أَينَ ما كانوا ۖ ثُمَّ يُنَبِّئُهُم بِما عَمِلوا يَومَ القِيامَةِ ۚ إِنَّ اللَّهَ بِكُلِّ شَيءٍ عَليمٌ

(Saheeh International)

Have you not considered that Allāh knows what is in the heavens and what is on the earth? There is in no private conversation three but that He is the fourth of them,¹ nor are there five but that He is the sixth of them – and no less than that and no more except that He is with them [in knowledge] wherever they are. Then He will inform them of what they did, on the Day of Resurrection. Indeed Allāh is, of all things, Knowing.

Through His knowledge of them and their secrets.