Musnad Ahmad 146:3

Yazid ibn Harun narrated to us, Ibn 'Awn informed us, from Nafi', from Ibn 'Umar, from 'Amir ibn Rabi'ah, from the Prophet, peace and blessings of Allah be upon him, who said: "If you see a funeral procession, stand up until it passes you," or he said: "Stop until it passes you." He said: "And Ibn 'Umar, when he saw a funeral procession, would stand until it passed him, and when he went out with a funeral procession, he would turn his back to the graves."