Musnad Abu Ya'la al-Mawsili 153:88

Ibrahim ibn Sa'id al-Jawhari narrated to us, Asram ibn Hawshab narrated to us, from Ziyad ibn Sa'd, from al-Zuhri, from Salim, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the shadow is a cubit and a half to two cubits, pray the Dhuhr prayer."