Sahih Muslim #1162 · The Book of Marriage

I used to read the Qur'an with my father in the vestibule (before the door of the mosque). When I recited the ayat (verses) concerning prostration, he prostrated himself. I said to him: Father, do you prostrate yourself in the path? He said: I heard Abu Dharr saying: I asked the Messenger of Allah (ﷺ) about the mosque that was first set up on the earth. He said: Masjid Harim. I said: Then which next? He said: The Masjid al-Aqsa. I said: How long is the space of time between the two? He said: Forty years. He (then) further said: The earth is a mosque for you, so wherever you are at the time of prayer, pray there.