(Imām Bukhārī narrates): 'Abdullāh ibn Maslamah narrated to us, from Imām Mālik, from 'Abdur-Rahmān ibn al-Qāsim, from 'Abdullāh ibn 'Abdullāh, he narrates that:
he saw 'Abdullāh ibn 'Umar, may Allāh be pleased with them both, sitting cross-legged in prayer, so I also did the same, and at that time I was young. So 'Abdullāh ibn 'Umar forbade me and told me that the sunnah in prayer is that you should raise your right foot and fold your left foot. I said: But you do it in this way? He said: My feet cannot bear my weight.
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 ـ رضى الله عنهما ـ 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.
Reference: Sahih Al Bukhari 827