Sahih Muslim
The Book of Vows (Kitab Al-Nadhr)
Book Number: 014 / Hadith Number: 4029
Anas reported that Allah's Apostle (may peace be upon him) saw an old man being supported between his two sons. He (the Holy Prophet) said: What is the matter with him? They said: He had taken the vow to walk (on foot to the Ka'ba). Thereupon he (Allah's Apoitle) said: Allah is indifferent to his inflicting upon himself chastisement, and he commanded him to ride.