Musannaf Ibn Abi Shayba 6:267

Ibn Habban (Hayyan) narrated to us, he said: Muhammad bin Salih narrated to us, he said: Yazid bin Zayd, the freed slave of Abu Usayd, narrated to me, from Abu Usayd, who said: I was with the Messenger of Allah, may Allah bless him and grant him peace, at the grave of Hamzah. The garment (namirah) was stretched over his head, but his feet were exposed. So it was stretched over his feet, but his head became exposed. The Messenger of Allah, may Allah bless him and grant him peace, said: "Place it over his head, and place some harmal (wild rue) plant over his feet."