Sahih Muslim Book 7, Hadith Number 3220.

Chapter : Do not undertake journey (purely for visit to the sacred places) but to three mosques.

Abu Haraira (Allah be pleased with him) reported Allah’s Messenger (way peace be upon him) as saying: One should undertake journey to three mosques: the mosque of the Ka’ba, my mosque, and the mosque of Elia (Bait al-Maqdis).