It is permissible for the Muhrim to wash his body and head

Ibrahim b. 'Abdullah narrated on the authorrity of his father that there cropped up a difference of opinion between Abdullah b. 'Abbas and al-Miswar b. Makhrama at a place (called) Abwa'. Abdullah b. 'Abbas contended that a Muhrim (is permitted) to wash his head, whereas Miswar contended that a Muhrim is not (permit- fed) to wash his head. So Ibn Abbas sent me (the father of Ibrabim) to Abu Ayyub al- Ansirl to ask him about it. (So I went to him) and found him taking bath behind two poles covered by a cloth. I gave him salutation, whereupon be asked: Who is this? I said: I am 'Abdullah b. Hunain. 'Abdullah b. 'Abbas has sent me to you to find out how the Messenger of Allah ﷺ washed his head in the state of Ihram. Abu Ayyub (Allah be pleased with him) placed his hand on the cloth and lowered it (a little) till his head became visible to me;
and he said to the man who was pouring water upon him to pour water. He poured water on his head. He then moved his head with the help of his hands and moved them (the hands) forward and backward and then said: This is how I saw him (the Messenger of Allah) doing.

وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَعَمْرٌو النَّاقِدُ، وَزُهَيْرُ بْنُ حَرْبٍ، وَقُتَيْبَةُ بْنُ سَعِيدٍ، قَالُوا حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ زَيْدِ بْنِ أَسْلَمَ، ح وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، - وَهَذَا حَدِيثُهُ - عَنْ مَالِكِ بْنِ أَنَسٍ، فِيمَا قُرِئَ عَلَيْهِ 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 عَبْدُ اللَّهِ بْنُ عَبَّاسٍ يَغْسِلُ الْمُحْرِمُ رَأْسَهُ . وَقَالَ الْمِسْوَرُ لاَ يَغْسِلُ الْمُحْرِمُ رَأْسَهُ . فَأَرْسَلَنِي ابْنُ عَبَّاسٍ إِلَى أَبِي أَيُّوبَ الأَنْصَارِيِّ أَسْأَلُهُ عَنْ ذَلِكَ فَوَجَدْتُهُ يَغْتَسِلُ بَيْنَ الْقَرْنَيْنِ وَهُوَ يَسْتَتِرُ بِثَوْبٍ - قَالَ - فَسَلَّمْتُ عَلَيْهِ فَقَالَ مَنْ هَذَا فَقُلْتُ أَنَا عَبْدُ اللَّهِ بْنُ حُنَيْنٍ أَرْسَلَنِي إِلَيْكَ عَبْدُ اللَّهِ بْنُ عَبَّاسٍ أَسْأَلُكَ كَيْفَ كَانَ رَسُولُ اللَّهِ ﷺ يَغْسِلُ رَأْسَهُ وَهُوَ مُحْرِمٌ فَوَضَعَ أَبُو أَيُّوبَ - رضى الله عنه - يَدَهُ عَلَى الثَّوْبِ فَطَأْطَأَهُ حَتَّى بَدَا لِي رَأْسُهُ ثُمَّ قَالَ لإِنْسَانٍ يَصُبُّ اصْبُبْ . فَصَبَّ عَلَى رَأْسِهِ ثُمَّ حَرَّكَ رَأْسَهُ بِيَدَيْهِ فَأَقْبَلَ بِهِمَا وَأَدْبَرَ ثُمَّ قَالَ هَكَذَا رَأَيْتُهُ ﷺ يَفْعَلُ .