In his famous book Fiqh-us-Sunnah, the late Egyptian scholar Sheikh SayyedSabiq, may Allah bless his soul, states: Scholars have unanimously agreed that Hajj is obligatory only once in a Muslim’s lifetime, unless he or she vows to perform extra Hajj, in which case he or she must fulfill his or her vow. Anything which is done over and above this is considered a voluntary or recommended act.
abu Hurayrah (may Allah be pleased with him) narrated that one day the Prophet (peace and blessings be upon him) addressed the people saying: “O people! Allah has prescribed Hajj upon you, so perform it.” A man asked, ‘Every year, O Messenger of Allah?’ The Prophet (peace and blessings be upon him) kept silent. When the man repeated his question thrice, the Prophet (peace and blessings be upon him) said: “Had I answered in the affirmative, it would have become a (yearly) obligation, and this would have been beyond your capacity.” Then he added: “Leave me alone so long as I leave you alone (i.e. do not bother me with questions about things I didn’t mention). What caused the destruction of the people prior to you was that they used to ask so many questions, and disagree with their Prophets. So when I command you to do something, you must obey me and do it to the extent of your ability, and if I forbid you from doing something, then you must avoid it.” (SahihMuslim, Book 7, Chapter 71, Hadith no. 3095)
Ibn `Abbas (may Allah be pleased with him) narrated: “Once the Prophet (peace and blessings be upon him) addressed us saying: “O People! Hajj has been prescribed for you.” At this Al-Aqra` IbnHabis got up and asked, “O Messenger of Allah! Are we to perform Hajj every year?” The Prophet (peace and blessings be upon him) said: “Had I answered in the affirmative, it would have become a (yearly) obligation, and had it become so, you would have failed to observe it. Hajj is obligatory only once in one’s lifetime. If one performs it more than once, it will be counted as a voluntary act.” (SunanAbu Dawud)