Hadith 501990
Chapter 3: Prayer - كتاب الصلاة
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، أَنَّهُ سَمِعَ عَبْدَ اللَّهِ بْنَ عُمَرَ، وَصَلَّى، إِلَى جَنْبِهِ رَجُلٌ فَلَمَّا جَلَسَ الرَّجُلُ فِي أَرْبَعٍ تَرَبَّعَ وَثَنَى رِجْلَيْهِ فَلَمَّا انْصَرَفَ عَبْدُ اللَّهِ عَابَ ذَلِكَ عَلَيْهِ فَقَالَ الرَّجُلُ فَإِنَّكَ تَفْعَلُ ذَلِكَ . فَقَالَ عَبْدُ اللَّهِ بْنُ عُمَرَ فَإِنِّي أَشْتَكِي .
Yahya related to me from Malik from Abdullah ibn Dinar that he had seen Abdullah ibn Umar with a man praying at his side. When the man sat in the fourth raka, heput both feet to one side and crossed them. When Abdullah finished, he disapproved of that to him, and the man protested, "But you do the same." Abdullah ibn Umar said, "I am ill."
Reference Muwatta Malik 501990
USC-MSA web (English) reference : Book 3, Hadith 52