Musnad Ahmad 30:131

Sufyan narrated to us, from al-Zuhri, from Sa’id, from Abu Hurairah, from the Prophet, peace and blessings of Allah be upon him, who said: "Journeys are not to be undertaken except to three mosques: the Sacred Mosque, my mosque, and the Al-Aqsa Mosque." Sufyan said: "And journeys are not to be undertaken except to three mosques, [they are] equal."