Musnad Ahmad 1157:2

Hammad ibn Khalid narrated to us, he said: Abdullah—meaning Ibn Umar—narrated to us, from Ibrahim ibn Muhammad, from his father, from Zaynab bint Jahsh, that she used to comb the hair of the Messenger of Allah, may Allah bless him and grant him peace. And she said once: "I used to comb the hair of the Messenger of Allah, may Allah bless him and grant him peace, in a basin made of brass."