Is it valid to perform the "greeting of the mosque" (تحية المسجد) and receive the reward for prayer as if it were a mosque, in a temporary location where Friday prayers are held but which is not endowed as a mosque? And is one sinful if they perform the "greeting of the mosque" therein, if the answer is no?

The "Tahiyyat al-Masjid" (greeting of the mosque) prayer is recommended for everyone who enters the mosque in a state of purity and intends to sit. This is supported by the hadith: "When one of you enters the mosque, he should not sit until he has prayed two rak'ahs." This applies by consensus to a mosque that has been endowed as a waqf.

As for a place designated for prayer but not endowed as a waqf, scholars have differed. The Malikis extended the ruling to include prayer rooms in homes. However, the Shafi'is stipulated for the validity of Tahiyyat al-Masjid that the mosque must be a waqf, and the land on which it is built must also be a waqf. If the land is held in monopoly or rented, it is not valid to be a mosque. Nevertheless, the building on such land can be validly endowed as a mosque, and Tahiyyat al-Masjid is valid therein. There is no sin upon one who prays Tahiyyat al-Masjid in these places.