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Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
bayhaqi:2776 – Abū ʿAbdullāh al-Ḥāfiẓ > Abū Zakariyyā Yaḥyá b. Muḥammad al-ʿAnbarī And ʾAbū Bakr Muḥammad b. Jaʿfar al-Muzakkī > Muḥammad / ān Ibn Ibrāhīm al-ʿAbdī > Ibn Bukayr > Mālik > ʿAbd al-Raḥman b. al-Qāsim > ʿUbaydullāh b. ʿAbdullāh b. ʿUmar

I saw ʿAbdullah bin ʿUmar crossing his legs while sitting in the prayer and I, a mere youngster in those days, did the same. Ibn ʿUmar forbade me to do so, and said, "The proper way is to keep the right foot propped up and bend the left in the prayer." I said questioningly, "But you are doing so (crossing the legs)." He said, "My feet cannot bear my weight." (Using translation from Bukhārī 827)   

البيهقي:٢٧٧٦ – أَخْبَرَنَا أَبُو عَبْدِ اللهِ الْحَافِظُ ثنا أَبُو زَكَرِيَّا يَحْيَى بْنُ مُحَمَّدٍ الْعَنْبَرِيُّ وَأَبُو بَكْرٍ مُحَمَّدُ بْنُ جَعْفَرٍ الْمُزَكِّي قَالَا ثنا مُحَمَّدٌ يَعْنِيَانِ ابْنَ إِبْرَاهِيمَ الْعَبْدِيَّ ثنا ابْنُ بُكَيْرٍ ثنا مَالِكٌ عَنْ عَبْدِ الرَّحْمَنِ بْنِ الْقَاسِمِ عَنْ عُبَيْدِ اللهِ بْنِ عَبْدِ اللهِ بْنِ عُمَرَ أَنَّهُ أَخْبَرَهُ أَنَّهُ كَانَ يَرَى عَبْدَ اللهِ بْنَ عُمَرَ يَتَرَبَّعُ فِي الصَّلَاةِ إِذَا جَلَسَ فَفَعَلْتُهُ وَأَنَا يَوْمَئِذٍ حَدِيثُ السِّنِّ فَنَهَانِي عَبْدُ اللهِ بْنُ عُمَرَ وَقَالَ إِنَّمَا سُنَّةُ الصَّلَاةِ

أَنْ تَنْصِبَ رِجْلَكَ الْيُمْنَى وَتَثْنِيَ الْيُسْرَى فَقُلْتُ إِنَّكَ تَفْعَلُ ذَلِكَ؟ فَقَالَ إِنَّ رِجْلِيَّ لَا تَحْمِلَانِي  

رَوَاهُ الْبُخَارِيُّ فِي الصَّحِيحِ عَنِ الْقَعْنَبِيِّ عَنْ مَالِكٍ

See similar narrations below:

Collected by Bukhārī
bukhari:827 – ʿAbdullāh b. Maslamah > Mālik > ʿAbd al-Raḥman b. al-Qāsim > ʿAbdullāh b. ʿAbdullāh > Kān

I saw ʿAbdullah bin ʿUmar crossing his legs while sitting in the prayer and I, a mere youngster in those days, did the same. Ibn ʿUmar forbade me to do so, and said, "The proper way is to keep the right foot propped up and bend the left in the prayer." I said questioningly, "But you are doing so (crossing the legs)." He said, "My feet cannot bear my weight."  

البخاري:٨٢٧ – حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ عَنْ مَالِكٍ عَنْ عَبْدِ الرَّحْمَنِ بْنِ الْقَاسِمِ عَنْ عَبْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ أَنَّهُ أَخْبَرَهُ أَنَّهُ كَانَ يَرَى عَبْدَ اللَّهِ بْنَ عُمَرَ ؓ يَتَرَبَّعُ فِي الصَّلاَةِ إِذَا جَلَسَ فَفَعَلْتُهُ وَأَنَا يَوْمَئِذٍ حَدِيثُ السِّنِّ فَنَهَانِي عَبْدُ اللَّهِ بْنُ عُمَرَ وَقَالَ إِنَّمَا سُنَّةُ الصَّلاَةِ

أَنْ تَنْصِبَ رِجْلَكَ الْيُمْنَى وَتَثْنِيَ الْيُسْرَى فَقُلْتُ إِنَّكَ تَفْعَلُ ذَلِكَ فَقَالَ إِنَّ رِجْلَىَّ لاَ تَحْمِلاَنِي