Malik related to me from Humayd ibn Qays from Ata ibn Abi Rabahthat 'Umar ibn alKhattab once asked Yala ibn Munya, who was pouringout water for him while he was having a ghusl, to pour some on hishead. Ya'la said, "Are you trying to make me responsible? I will onlypour it out if you tell me to do so." Umar ibn al-Khattab said, "Pour.It will only make (my head) more unkempt."
USC-MSA web (English) reference: Book 20, Hadith 5
Arabic reference: Book 20, Hadith 712