Maliks Muwatta Book 3, Hadith Number 52.
Section : Behaviour in the Sitting in the Prayer.
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, he put 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.”