Maliks Muwatta Book 20, Hadith Number 94.
Section : Things that Someone in Ihram is Allowed to do.
Yahya related to me from Malik from Alqama ibn Abi Alqama that his mother said, “I heard A’isha, the wife of the Prophet, may Allah bless him and grant him peace, being asked whether some one in ihram could scratch their body or not, and she said, ‘Yes, he can scratch it and do so as hard as he pleases. I would scratch even if my hands were tied and I could only use my feet.'”