Musnad Abu Ya'la al-Mawsili 117:150

Muhammad ibn al-Minhal al-Darir narrated to us, Yazid narrated to us, from Humayd, from Thabit, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) saw a man being supported between his two sons. He said, "What is the matter with him?" They said, "He vowed to perform Hajj walking." He said, "Indeed, Allah is in no need of this man's walking, so let him ride."