Musnad Ahmad 31:882

Abu Mu'awiyah narrated to us, Laith narrated to us, from Shahr, who said: We met Abu Sa'id while we were intending to go to At-Tur, and he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Do not undertake a journey except to three mosques: the Sacred Mosque, the Mosque of Medina, and Bayt al-Maqdis."