Musannaf Ibn Abi Shayba 3:4353

Waki’ narrated to us, he said: Sufyan and Shu’bah narrated to us, from ‘Abd al-Malik ibn ‘Umayr, from Musa ibn Talhah, from ‘Uthman, that he used to perform one rak’ah as witr. And he would say: "I liken it to nothing but the stray camel."