Where to place the fingers when bowing

It was narrated that 'Uqbah bin 'Amir said:

"Shall I not show you how I saw the Messenger of Allah ﷺ pray?" We said: "Yes." So he stood up and when he bowed, he placed his palms on his knees and put his fingers behind his knees, and held his arms out from his sides, until every part of him settled. Then he raised his head and stood up until every part of him settled. Then he prostrated and held his arms out from his sides, until every part of him settled. Then he sat up until every part of him settled. Then he prostrated again until every part of him settled. Then he did four rak'ahs like that. Then he said: "This is how I saw the Messenger of Allah ﷺ pray, and this is how he used to lead us in prayer."

أَخْبَرَنَا أَحْمَدُ بْنُ سُلَيْمَانَ الرَّهَاوِيُّ، قَالَ حَدَّثَنَا حُسَيْنٌ، عَنْ زَائِدَةَ، عَنْ عَطَاءٍ، عَنْ سَالِمٍ أَبِي عَبْدِ اللَّهِ، عَنْ عُقْبَةَ بْنِ عَمْرٍو، قَالَ أَلاَ أُصَلِّي لَكُمْ كَمَا رَأَيْتُ رَسُولَ اللَّهِ ﷺ يُصَلِّي فَقُلْنَا بَلَى . فَقَامَ فَلَمَّا رَكَعَ وَضَعَ رَاحَتَيْهِ عَلَى رُكْبَتَيْهِ وَجَعَلَ أَصَابِعَهُ مِنْ وَرَاءِ رُكْبَتَيْهِ وَجَافَى إِبْطَيْهِ حَتَّى اسْتَقَرَّ كُلُّ شَىْءٍ مِنْهُ ثُمَّ رَفَعَ رَأْسَهُ فَقَامَ حَتَّى اسْتَوَى كُلُّ شَىْءٍ مِنْهُ ثُمَّ سَجَدَ فَجَافَى إِبْطَيْهِ حَتَّى اسْتَقَرَّ كُلُّ شَىْءٍ مِنْهُ ثُمَّ قَعَدَ حَتَّى اسْتَقَرَّ كُلُّ شَىْءٍ مِنْهُ ثُمَّ سَجَدَ حَتَّى اسْتَقَرَّ كُلُّ شَىْءٍ مِنْهُ ثُمَّ صَنَعَ كَذَلِكَ أَرْبَعَ رَكَعَاتٍ ثُمَّ قَالَ هَكَذَا رَأَيْتُ رَسُولَ اللَّهِ ﷺ يُصَلِّي وَهَكَذَا كَانَ يُصَلِّي بِنَا .