Jazaak Allah khair
Rasulullah (saw) said, when a person passes away, all his actions terminate
but three; knowledge through which people benefit, sadaqa Jaariyya(recurring)
and a pious child who makes dua for the parents. Building a musjid on behalf
of the mother will certainly be sadaqa Jaariyya as people will perform salaah
and other righteous deeds there. Your mother will be rewarded for all that,
Insha-Allah.
And Allah Ta’ala knows best
Mufti E. Desai