It is narrated by the Tabi'i Hasan al-Basri (Rahimahullah): "I saw Uthman bin Affan (Radi Allahu Anhu) taking a midday nap (Qailulah) in the mosque while covering himself with a sheet; there was no one around him, and he was the Ameer-ul-Mu'minin (Leader of the Faithful) of the time!"

(Fada'il al-Sahabah li-Ahmad, 800)