Musnad Ahmad 769:12

'Abd al-Rahman—who is Ibn Mahdi—narrated to us, Shu'bah narrated to us, from a sheikh from Bajilah, who said: I heard Ibn Abi Awfa saying: Abu Bakr, may Allah be pleased with him, asked permission to enter upon the Prophet, may Allah bless him and grant him peace, while there was a slave girl with him beating a tambourine. So he entered. Then Umar, may Allah be pleased with him, asked permission to enter, so he entered. Then Uthman, may Allah be pleased with him, asked permission to enter, so she stopped. He said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, Uthman is a shy man."