Sayyiduna al-Hasan bin Ali (may Allah be pleased with him) said: "The Messenger of Allah (peace and blessings be upon him) did both. He entered into ihram (began the talbiyah) when his mount stood upright with him, and he also entered into ihram while he was on the ground at al-Bayda', before his mount stood upright with him."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2739