Musnad Ahmad 32:1524

Muhammad ibn Abdullah narrated to us, Humayd narrated to us, Thabit narrated to us, he said: Anas said: An old man passed by, being supported between his two sons. He said: He said: "What is the matter with this one?" They said: "He made a vow, O Messenger of Allah, that he would walk." He said: "Indeed, Allah is in no need of this man torturing himself." So he commanded him to ride, and he rode.