Musannaf Ibn Abi Shayba 27:389

Muhammad ibn Bishr narrated to us, he said: Sadaqah ibn al-Muthanna narrated to us, he said: I heard my grandfather, Rabah ibn al-Harith, from Sa'id ibn Zayd, he said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "Uthman is in Paradise."