Exodus 30:11-16

11 And the Lord said to Moses,
12 When you are taking the number of the children of Israel, let every man who is numbered give to the Lord a price for his life, so that no disease may come on them when they are numbered.
13 And this is what they are to give; let every man who is numbered give half a shekel, by the scale of the holy place: (the shekel being valued at twenty gerahs:) this money is an offering to the Lord.
14 Everyone who is numbered, from twenty years old and over, is to give an offering to the Lord.
15 The man of wealth is to give no more and the poor man no less than the half-shekel of silver, when the offering is made to the Lord as the price for your lives.
16 And you are to take this money from the children of Israel to be used for the work of the Tent of meeting, to keep the memory of the children of Israel before the Lord and to be the price of your lives.
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