(10) The Rites of Pilgrimage
كتاب المناسك
(12a) Chapter: What one who is in the Sacred State must avoid - Section 1
(11) باب ما يجتنبه المحرم
Mishkat al-Masabih 2684 
Abu Ayyub said that the Prophet used to wash his head when he was in the sacred state. Bukhari and Muslim.
وَعَن أَبِي أَيُّوبَ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يَغْسِلُ رَأْسَهُ وَهُوَ مُحْرِمٌ
 مُتَّفق عَلَيْهِ  (الألباني)حكم :
Reference : Mishkat al-Masabih 2684
In-book reference : Book 10, Hadith 174