Musnad Abu Ya'la al-Mawsili 6:130

'Abd al-A'la ibn Hammad al-Narsi narrated to us, Wahb ibn Jarir narrated to us, my father narrated to us, he said: I heard Muhammad ibn Ishaq narrating from Abu al-Zinad, from 'A'ishah bint Sa'd, from Sa'd, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) took the road of al-Far', he would enter into the state of Ihram when his riding beast stood up with him. And when he took the other road, he would enter into the state of Ihram when he ascended the high ground of al-Bayda'.