Exodus 30:13-15

13Everyone who crosses over to those who are numbered is to pay this: a half shekel according to the shekel of the sanctuary (a shekel weighs twenty gerahs). The half shekel is to be an offering to the Lord.

14Everyone who crosses over to those numbered, from twenty years old and up, is to pay an offering to the Lord.

15The rich are not to increase it, and the poor are not to pay less than the half shekel when giving the offering of the Lord, to make atonement for your lives.