حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ إِبْرَاهِيمَ بْنِ عَبْدِ اللَّهِ بْنِ حُنَيْنٍ، عَنْ أَبِيهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ، وَالْمِسْوَرَ بْنَ مَخْرَمَةَ، اخْتَلَفَا بِالأَبْوَاءِ فَقَالَ عَبْدُ اللَّهِ يَغْسِلُ الْمُحْرِمُ رَأْسَهُ ‏.‏ وَقَالَ الْمِسْوَرُ بْنُ مَخْرَمَةَ لاَ يَغْسِلُ الْمُحْرِمُ رَأْسَهُ ‏.‏ قَالَ فَأَرْسَلَنِي عَبْدُ اللَّهِ بْنُ عَبَّاسٍ إِلَى أَبِي أَيُّوبَ الأَنْصَارِيِّ فَوَجَدْتُهُ يَغْتَسِلُ بَيْنَ الْقَرْنَيْنِ وَهُوَ يُسْتَرُ بِثَوْبٍ فَسَلَّمْتُ عَلَيْهِ فَقَالَ مَنْ هَذَا فَقُلْتُ أَنَا عَبْدُ اللَّهِ بْنُ حُنَيْنٍ أَرْسَلَنِي إِلَيْكَ عَبْدُ اللَّهِ بْنُ عَبَّاسٍ أَسْأَلُكَ كَيْفَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَغْسِلُ رَأْسَهُ وَهُوَ مُحْرِمٌ قَالَ فَوَضَعَ أَبُو أَيُّوبَ يَدَهُ عَلَى الثَّوْبِ فَطَأْطَأَهُ حَتَّى بَدَا لِي رَأْسُهُ ثُمَّ قَالَ لإِنْسَانٍ يَصُبُّ عَلَيْهِ اصْبُبْ ‏.‏ فَصَبَّ عَلَى رَأْسِهِ ثُمَّ حَرَّكَ رَأْسَهُ بِيَدَيْهِ فَأَقْبَلَ بِهِمَا وَأَدْبَرَ ثُمَّ قَالَ هَكَذَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَفْعَلُ ‏.‏

Yahya related to me from Malik from Zayd ibn Aslam from Ibrahimibn Abdullah ibn Hunayn from his father Abdullah ibn Hunayn thatAbdullah ibn Abbas and al-Miswar ibn Makhrama once had a disagreementat al-Abwa. Abdullah said that some one in ihram could wash his head,and al Miswar ibn Makhrama maintained that some one in ihram could notwash his head. Abdullah ibn Hunayn continued, "Abdullah ibnAbbas sent me to Abu Ayyub al-Ansari, and I found him doing ghuslbetween the posts of a well, screened by a garment. I greeted him andhesaid, 'Who is that?' I replied, 'I am 'Abdullah ibn Hunayn.'Abdullah ibn Abbas sent me to you to ask how the Messenger of Allah,may Allah bless him and grant him peace, used to wash his head when hewas in ihram.' " He continued, "Abu Ayyub put his hand on thegarment and pulled it down until I could see his head. He said to theman who was pouring out the water for him, 'Pour,' and he poured someover his head. Then he passed his hands over his head from the frontto the back and then to the front again, and then said, 'I saw theMessenger of Allah, may Allah bless him and grant him peace, doing itlike this.' " Translation Not Available

Book reference: Muwatta Imam Malik Book 20 Hadith 711
Web reference: Muwatta Imam Malik Book 20 Hadith 4