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