Ya'la bin Umayyah (may Allah be pleased with him) said: Umar bin al-Khattab (may Allah be pleased with him) was bathing behind the screen of a camel, and on the other side I was screening him with a cloth. Umar (may Allah be pleased with him) said: Pour water over my head, O Ya'la! I said: The Commander of the Believers knows best. Umar (may Allah be pleased with him) said: By Allah! Water only makes the hair more dishevelled. Then he mentioned the name of Allah and poured water over his head.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9133