Musnad Ahmad 288:24

'Affan narrated to us, he said: Shu'bah narrated to us, he said: Nu'man ibn Salim narrated to us, he said: I heard so-and-so, or his grandfather, say: My grandfather used to say to me, while he was in prayer, gesturing to me: "Hand me the two sandals." So I would hand them to him, and he would put them on and pray in them, and he would say: "I saw the Messenger of Allah (peace and blessings of Allah be upon him) praying in his two sandals."