Maliks Muwatta Book 20, Hadith Number 18.
Section : Veiling the Face while in Ihram.
Yahya related to me from Malik from Humayd ibn Qays from Ata ibn Rabah that a bedouin came to the Messenger of Allah, may Allah bless him and grant him peace, when he was at Hunayn, and he was wearing a shirt with traces of yellow on it. He said, “Messenger of Allah, I have entered ihram for umra. What should I do?” The Messenger of Allah, may Allah bless him and grant him peace, said to him, “Take off your shirt and wash off this yellowness and do in umra as you would do on hajj.”