كتاب المناسك
10  
The Rites of Pilgrimage
(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