Musannaf Ibn Abi Shayba 3:595

Waki' narrated to us, he said: I heard al-A'mash saying: I saw Anas ibn Malik in Mecca standing, praying near the Kaaba, and I did not cross in front of him. He said: He was standing, praying, balanced in his prayer. When he raised his head [from prostration], he would stand upright until the folds of his stomach became level.