Musannaf Ibn Abi Shayba 3:4684

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from al-A'mash, from al-Minhal, from Abu Ubaydah, he said: Abdullah saw a man praying while keeping his feet together. He said: "If this person were to alternate between his feet, it would have been better."