What is built in return
One condition is attached and it is not about the size.
مَنْ بَنَى مَسْجِدًا لِلَّهِ كَمَفْحَصِ قَطَاةٍ أَوْ أَصْغَرَ بَنَى اللَّهُ لَهُ بَيْتًا فِي الْجَنَّةِ
Man banā masjidan lillāhi ka-mafḥaṣi qaṭātin aw aṣghar, bana-Llāhu lahu baytan fi-l-jannah
“Whoever builds a mosque for Allah, even the size of a sandgrouse's scrape in the ground or smaller, Allah builds him a house in the Garden.”
Where this comes from
A sandgrouse makes its nest by scratching a shallow dip in the ground. It is the smallest structure the desert offered as a comparison, and the return on it is a house.
The full narration
It was narrated from Jabir bin 'Abdullah that:The Messenger of Allah said: "Whoever builds a mosque for the sake of Allah, like a sparrow's nest for Allah or even smaller, Allah will build for him a house in Paradise
Source
Sunan Ibn Mājah · 738
- Al-Albani: Sahih
- Muhammad Fouad Abd al-Baqi: Sahih
- Shuaib Al Arnaut: Sahih
- Zubair Ali Zai: Isnaad Sahih