It was narrated from 'Abdullah bin 'Abbas and Al-Miswar bin Makhramah that:

they had a difference of opinion in al-Abwa. Ibn 'Abbas said: "The Muhrim (Pilgrim in Ihram) may wash his head." Al-Miswar said: "He should not wash his head."Ibn 'Abbas sent me (the narrator) to Abu Ayyub Al-Ansari to ask him about that. I found him performing Ghusl in front of the well, screened with a cloth. I greeted him with Salam and said: "Abdullah bin 'Abbas has sent me to you to ask you how the Messenger of Allah used to wash his head when he was in Ihram." Abu put his hand on the cloth and lowered it, until his head appeared, then he told someone to puor water on his head. Then he rubbed his head with his hands, back and forth, and said: "This is what I saw the Messenger of Allah do."

أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، عَنْ إِبْرَاهِيمَ بْنِ عَبْدِ اللَّهِ بْنِ حُنَيْنٍ، عَنْ أَبِيهِ، عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ، وَالْمِسْوَرِ بْنِ مَخْرَمَةَ، أَنَّهُمَا اخْتَلَفَا بِالأَبْوَاءِ فَقَالَ ابْنُ عَبَّاسٍ يَغْسِلُ الْمُحْرِمُ رَأْسَهُ . وَقَالَ الْمِسْوَرُ لاَ يَغْسِلُ رَأْسَهُ . فَأَرْسَلَنِي ابْنُ عَبَّاسٍ إِلَى أَبِي أَيُّوبَ الأَنْصَارِيِّ أَسْأَلُهُ عَنْ ذَلِكَ فَوَجَدْتُهُ يَغْتَسِلُ بَيْنَ قَرْنَىِ الْبِئْرِ وَهُوَ مُسْتَتِرٌ بِثَوْبٍ فَسَلَّمْتُ عَلَيْهِ وَقُلْتُ أَرْسَلَنِي إِلَيْكَ عَبْدُ اللَّهِ بْنُ عَبَّاسٍ أَسْأَلُكَ كَيْفَ كَانَ رَسُولُ اللَّهِ ﷺ يَغْسِلُ رَأْسَهُ وَهُوَ مُحْرِمٌ فَوَضَعَ أَبُو أَيُّوبَ يَدَهُ عَلَى الثَّوْبِ فَطَأْطَأَهُ حَتَّى بَدَا رَأْسُهُ ثُمَّ قَالَ لإِنْسَانٍ يَصُبُّ عَلَى رَأْسِهِ ثُمَّ حَرَّكَ رَأْسَهُ بِيَدَيْهِ فَأَقْبَلَ بِهِمَا وَأَدْبَرَ وَقَالَ هَكَذَا رَأَيْتُ رَسُولَ اللَّهِ ﷺ يَفْعَلُ .