Mishkat al-Masabih 753

Abu Dharr said that he asked God’s Messenger which mosque was set up first in the earth and was told that it was the sacred mosque. He asked which came next and was told that it was the Aqsa mosque. He asked how long a space of time separated their building and the Prophet (ﷺ) told him it was forty years, adding, “Then the earth is a mosque for you, so pray wherever you are at the time of prayer.” (Bukhari and Muslim.)