Sayyidah Aishah (may Allah be pleased with her) says: The Messenger of Allah (peace and blessings be upon him) would first wash his hands, then perform ablution as for prayer, then put his palms into the water and run them through the roots of his hair, until, when he (peace and blessings be upon him) knew that the water had reached the skin, he would scoop three handfuls with his hand and pour them over his head, then wash (the rest of his body).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 819