Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: If a person, in the ritual bath for janabah, washes his head with khitmi (marshmallow) and washes it well, it will do him no harm if he does not pour water over it.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and there is no narrator in its chain who has been declared da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1483