Musnad Ahmad 30:1855

'Affan narrated to us, 'Abd al-Warith narrated to us, Muhammad ibn al-Munkadir narrated to us, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "If one of you is sitting in the sun, and it (the shade) recedes from him, then let him move from his sitting place."