Musnad Ahmad 288:14

Bahz narrated to us, Shu'bah narrated to us, an-Nu'man ibn Salim narrated to us, from a man, whose grandfather was Aws ibn Abi Aws, that he was praying and gesturing toward his sandals while he was in the prayer. He would take them and put them on, and pray in them. And he would say: "The Messenger of Allah (peace and blessings of Allah be upon him) used to pray in his sandals."