Musnad Abu Ya'la al-Mawsili 117:258

Zuhayr narrated to us, Abdullah bin Bakr narrated to us, Humayd narrated to us, from Thabit, from Anas, from the Prophet (peace be upon him): That he saw an old man being supported between his two sons. He said, "What is the matter with this one?" They said, "O Messenger of Allah, he vowed to walk." He said, "Indeed, Allah is free from the need of this one torturing himself." He then commanded him, so he rode.