Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 3095
Abu Huraira (Allah be pleased with him) reported: Allah's Messenger (may peace be upon him) addressed us and said: O people, Allah has made Hajj obligatory for you; so perform Hajj. Thereupon a person said: Messenger of Allah, (is it to be performed) every year? He (the Holy Prophet) kept quiet, and he repeated (these words) thrice, whereupon Allah's Messenger (may peace be upon him) said: If I were to say" Yes," it would become obligatory (for you to perform it every year) and you would not be able to do it. Then he said: Leave me with what I have left to you, for those who were before you were desroyed because of excessive questioning, and their opposition to their apostles. So when I command you to do anything, do it as much as it lies in your power and when I forbid you to do anything, then abandon it.