Maliks Muwatta Book 9, Hadith Number 81.

Section : How to Do the Prayer in General.

Yahya related to me from Malik from Yahya ibn Said from Muhammad ibn Yahya ibn Habban that his paternal uncle Wasi ibn Habban said, “I was praying, and Abdullah ibn Umar was resting his back on the wall of the qibla. When I had finished the prayer I turned towards him on my left hand side. Abdullah ibn Umar said, ‘What stopped you from turning away to your right?’ I replied, ‘I saw you and turned towards you.’ Abdullah said, ‘You have spoken correctly. People say that you should turn away to your right, but when you pray, you can turn whichever way you wish. If you like, to your right, and if you like, to your left.'”