Whenever Ibn ʿUmar entered the Kaʿba he used to walk straight keeping the door at his back on entering, and used to proceed on till about three cubits from the wall in front of him, and then he would offer the prayer there aiming at the place where Messenger of Allah ﷺ prayed, as Bilal had told him. There is no harm for any person to offer the prayer at any place inside the Kaʿba.  

أَنَّهُ كَانَ إِذَا دَخَلَ الْكَعْبَةَ مَشَى قِبَلَ الْوَجْهِ حِينَ يَدْخُلُ وَيَجْعَلُ الْبَابَ قِبَلَ الظَّهْرِ يَمْشِي حَتَّى يَكُونَ بَيْنَهُ وَبَيْنَ الْجِدَارِ الَّذِي قِبَلَ وَجْهِهِ قَرِيبًا مِنْ ثَلاَثِ أَذْرُعٍ فَيُصَلِّي يَتَوَخَّى الْمَكَانَ الَّذِي أَخْبَرَهُ بِلاَلٌ أَنَّ رَسُولَ اللَّهِ ﷺ صَلَّى فِيهِ وَلَيْسَ عَلَى أَحَدٍ بَأْسٌ أَنْ يُصَلِّيَ فِي أَىِّ نَوَاحِي الْبَيْتِ شَاءَ  

[Machine] He asked Bilal where did the Messenger of Allah ﷺ pray? Meaning, in the Kaaba. So Bilal showed him where he prayed, and he did not ask him how many prayers he performed. Ibn Umar, when he entered the house, would face its front and put the door behind his back, then he would walk until he was close to the wall, about the distance of three cubits, then he would pray, choosing the place that Bilal informed him that the Messenger of Allah ﷺ used to pray in.  

أَنَّهُ سَأَلَ بِلَالًا أَيْنَ صَلَّى رَسُولُ اللهِ ﷺ؟ يَعْنِي فِي الْكَعْبَةِ فَأَرَاهُ بِلَالٌ حَيْثُ صَلَّى وَلَمْ يَسْأَلْهُ كَمْ صَلَّى وَكَانَ ابْنُ عُمَرَ إِذَا دَخَلَ الْبَيْتَ مَشَى قِبَلَ وَجْهِهِ وَجَعَلَ الْبَابَ قِبَلَ ظَهْرِهِ ثُمَّ مَشَى حَتَّى يَكُونَ بَيْنَهُ وَبَيْنَ الْجِدَارِ قَرِيبٌ مِنْ ثَلَاثَةِ أَذْرُعٍ ثُمَّ صَلَّى يَتَوَخَّى الْمَكَانَ الَّذِي أَخْبَرَهُ بِلَالٌ أَنَّ رَسُولَ اللهِ ﷺ صَلَّى فِيهِ