Musannaf Ibn Abi Shayba 3:3758

Abu Bakr narrated to us, he said: Jarir ibn Hazim narrated to us, from al-Mughirah ibn Hakim al-San'ani, he said: I saw Ibn Umar sitting cross-legged at the end of his prayer when he raised his head from the final prostration. When he finished praying, I asked him [about it], and he said: "I have a complaint in my leg."