Musnad Ahmad 32:1311

Hammad ibn Khalid narrated to us, Malik narrated to us, Ziyad ibn Sa’d narrated to us, from al-Zuhri, from Anas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) let his forelock hang down as long as Allah willed for him to let it hang down, then he parted it afterward."