Maliks Muwatta Book 20, Hadith Number 5.

Section : The Ghusl of Someone in Ihram.

Malik related to me from Humayd ibn Qays from Ata ibn Abi Rabah that ‘Umar ibn alKhattab once asked Yala ibn Munya, who was pouring out water for him while he was having a ghusl, to pour some on his head. Ya’la said, “Are you trying to make me responsible? I will only pour it out if you tell me to do so.” Umar ibn al-Khattab said, “Pour. It will only make (my head) more unkempt.”