The Prophet (peace be upon him) said:

“Whoever builds a mosque for Allah, Allah will build for him likewise in Paradise.”

[Sahîh al-Bukhârî and Sahîh Muslim]

In some narrations of this hadith, it reads “Whoever builds a mosque seeking Allah’s pleasure…”

It is also narrated that the Prophet (peace be upon him) said:

“Whoever builds a mosque for Allah – be it large or small – Allah will build for him a house in Paradise.”

[Sunan al-Tirmidhî]

In a narration in Musnad al-Bazzâr:

“Whoever builds a mosque for Allah – though it be the size of the ground nest of a sandgrouse – Allah will build for him a house in Paradise.”
The building of a mosque is a way for a person to continue to earn blessings after death.

The Prophet (peace be upon him) said:

“Among what continues to accrue for a believer of his good works after death are the following: knowledge that he learned and then imparted to others, a pious child whom he left behind, a copy of the Qur’ân that he bequeathed, a mosque that he built, a guest house he built for travelers, a river that he made to flow, and charity he spends from his wealth when he is in good health – all of this continues to avail him after his death.”

[Sunan Ibn Majah and Sunan al-Bayhaqî]