Musnad Ahmad 32:1922

Affan narrated to us, Hammad narrated to us, Humayd and Thabit informed us, from Anas: That 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 this?" They said: "O Messenger of Allah, he vowed to perform Hajj walking." He said: "Indeed, Allah is in no need of him torturing himself; let him ride."