Musnad Abu Ya'la al-Mawsili 159:106

Yahya ibn Ayyub narrated to us, Ismail narrated to us, he said: Amr informed me, from Abd al-Rahman al-A'raj, from Abu Hurayrah, that the Prophet (peace and blessings of Allah be upon him) encountered an old man walking between his two sons, leaning upon them. The Prophet (peace and blessings of Allah be upon him) said: "What is the matter with this old man?" His two sons said: "O Messenger of Allah, he had a vow." The Prophet (peace and blessings of Allah be upon him) said: "Ride, O old man, for Allah, the Mighty and Majestic, is free of need of you and your vow."