The Prophet ﷺ said, "Whoever builds a mosque for the sake of Allah, even one as small as the nesting hollow a sandgrouse scrapes for its eggs, Allah will build for him a house in Paradise."
حدثنا محمد بن جعفر حدثنا شعبة عن جابر عن عمار عن سعيد بن جبير
عن النبي ﷺ أنه قال من بنى لله مسجدا ولو كمفحص قطاة لبيضها بنى الله له بيتا في الجنة
Its isnad is daif due to the weakness of Jabir Jafi. Ammar is Ibn Muawiya Dahni. This is found in Majma al-Zawaid 2/7, where he said, Ahmad and Bazzar narrated it, and it contains Jabir Jafi, who is daif. The meaning of the hadith is sahih, narrated by Ibn Majah and by Ibn Khuzayma in his Sahih, from the hadith of Jabir bin Abdullah. See Targhib wal-Tarhib 1/117.Mafhas al-qataah is the spot where the sandgrouse crouches and lays its eggs, as if scraping away the dirt from it, meaning uncovering it. Al-fahs means searching and uncovering, as stated in Nihaya.