كتاب الحيض
3  
The Book of Menstruation
(3)
Chapter: It is permissible for a menstruating woman to wash her husband’s head, and comb his hair; her leftovers are pure (tahir); and regarding reclining in her lap and reciting Qur’an
(3)
باب جَوَازِ غَسلِ الْحَائِضِ رَأْسَ زَوْجِهَا وَتَرْجِيلِهِ وَطَهَارَةِ سُؤْرِهَا وَالاِتِّكَاءِ فِي حِجْرِهَا وَقِرَاءَةِ الْقُرْآنِ فِيهِ
Sahih Muslim 297 e

Al-Aswad narrated it from 'A'isha that she observed:

I used to wash the head of the Messenger of Allah (ﷺ), while I was in a state of menstruation.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا حُسَيْنُ بْنُ عَلِيٍّ، عَنْ زَائِدَةَ، عَنْ مَنْصُورٍ، عَنْ إِبْرَاهِيمَ، عَنِ الأَسْوَدِ، عَنْ عَائِشَةَ، قَالَتْ كُنْتُ أَغْسِلُ رَأْسَ رَسُولِ اللَّهِ صلى الله عليه وسلم وَأَنَا حَائِضٌ ‏.‏
Reference : Sahih Muslim 297e
In-book reference : Book 3, Hadith 10
USC-MSA web (English) reference : Book 3, Hadith 586
  (deprecated numbering scheme)