Musnad Abu Ya'la al-Mawsili 110:187

I mentioned to Ibn Abbas the *ihlal* (entering into the state of Ihram) of the Messenger of Allah, may Allah bless him and grant him peace. He said: The Messenger of Allah, may Allah bless him and grant him peace, made the Ihram obligatory when he finished his prayer. Then he went out, and when he mounted his riding beast and it stood upright with him, he pronounced the *ihlal*. A group of people perceived that from him and said: "He pronounced the *ihlal* when his riding beast rose with him," and that is because they did not perceive anything other than that. Then he traveled until he ascended the *Bayda* (a location), and he pronounced the *ihlal*. Men who were with him perceived it and said: "He pronounced the *ihlal* when he ascended the *Bayda*."