Musnad Ahmad 32:1644

Qutaybah narrated to us, he said: Ibn Lahi'ah informed us, from Khalid ibn Abi 'Imran, from Sa'd ibn Ishaq ibn Ka'b ibn 'Ujrah, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Change the gray hair, and do not approach black."