(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.
Reference: Sahih Al Bukhari 827