Is zakat due only on the principal amount inherited from the mother, or on the principal amount plus the returns from bank certificates in which the father has invested this money?

Bank investment certificates with a fixed, guaranteed return are usurious loans forbidden [in Islam]. The usurious interest is not owned by the account holder, and zakat is not obligatory upon it. Rather, all of it must be spent on public welfare and distributed to the poor and needy. Zakat is obligatory on the principal of the lawful money. If the owner or their family are poor, they may take from it what suffices their need.