Is it true that whoever builds a mosque for Allah – even if it is the size of a bird's nest – Allah will build for him a house in Paradise, and is it permissible to designate a small place in a room for prayer with the intention of receiving the reward associated with treating it as a mosque?

The Prophet, peace and blessings be upon him, encouraged the building of mosques. Uthman, may Allah be pleased with him, narrated that the Messenger of Allah, peace and blessings be upon him, said: "Whoever builds a mosque for Allah, Allah will build for him a similar house in Paradise." Some narrations add: "even if it be like the nest of a sandgrouse," meaning the place a sandgrouse makes for laying its eggs.

As for dedicating a place for prayer in one's home, this is permissible and was practiced by the Companions and their successors, such as Ammar ibn Yasir and Itban ibn Malik. Scholars have stated that such a place does not take on the rulings of a dedicated mosque (masjid musabbal); thus, it is not subject to continuous purity, nor are major ritual impurity (janabah) or menstruation forbidden within it.

Nevertheless, praying in these places does not equate to the virtue of praying in dedicated mosques, nor does the virtue of building mosques mentioned in the hadith apply to them.