Musnad Ahmad 32:340

Abu Amir narrated to us, Abd al-Rahman ibn Abi al-Mawal narrated to us, from Musa ibn Ibrahim ibn Abi Rabi'ah, from his father, who said: We entered upon Anas ibn Malik while he was praying in a single garment, wrapped in it, with his rida' (outer garment) placed aside. I said to him: "Do you pray in a single garment?" He said: "I saw the Messenger of Allah, may Allah bless him and grant him peace, praying like this."