Abdullah bin Abbas and Miswar bin Makhrama had a disagreement at al-Abwa.Ibn Abbas said, "A person in the state of ihram may wash his head."Miswar said, "A person in ihram may not wash his head."So Abdullah bin Abbas sent me to Abu Ayub al-Ansari, and I found him bathing between the two posts of the well, screened by a cloth. I greeted him.He asked, "Who is this?"I replied, "I am Abdullah bin Hunayn. Abdullah bin Abbas has sent me to ask you how the Messenger of Allah ﷺ used to wash his head while in ihram."Abu Ayub placed his hand on the cloth and lowered it until I could see his head. Then he said to a man who was pouring water for him, "Pour."The man poured water over his head, then Abu Ayub moved his head with his hands, bringing them forward and back. Then he said, "This is how I saw him ﷺ do it."
حدثنا عبد الله بن مسلمة عن مالك عن زيد بن أسلم
أن عبد الله بن عباس والمسور بن مخرمة اختلفا بالأبواء فقال ابن عباس يغسل المحرم رأسه وقال المسور لا يغسل المحرم رأسه فأرسله عبد الله بن عباس إلى أبي أيوب الأنصاري فوجده يغتسل بين القرنين وهو يستر بثوب قال فسلمت عليه قال من هذا قلت أنا عبد الله بن حنين أرسلني إليك عبد الله بن عباس أسألك كيف كان رسول الله ﷺ يغسل رأسه وهو محرم قال فوضع أبو أيوب يده على الثوب فطأطأه حتى بدا لي رأسه ثم قال لإنسان يصب عليه اصبب قال فصب على رأسه ثم حرك أبو أيوب رأسه بيديه فأقبل بهما وأدبر ثم قال هكذا رأيته يفعل ﷺ
Bukhari narrated it in "Recompense for Hunting," chapter "Bathing for One in Muharram," no. 1840, and Muslim in "Hajj," chapter "The Permissibility of One in Muharram Washing His Body and Head," no. 1205, from the hadith of Malik through this route, and it is found in Muwatta (Yahya): 1/323.