Musnad Ahmad 26:274

Muhadir Abu al-Muwarri' narrated to us, 'Asim narrated to us, from Awsajah ibn al-Rimmah, from 'Abdullah ibn Abi al-Hudhayl, from Ibn Mas'ud, that the Messenger of Allah (peace and blessings of Allah be upon him) used to say: "O Allah, You have made my creation excellent, so make my character excellent."