Exodus 34:20
But the firstling of an ass thou shalt redeem with a lamb: and if thou redeem him not, then shalt thou break his neck. All the firstborn of thy sons thou shalt redeem. And none shall appear before me empty. But the firstling of an ass thou shalt redeem with a lamb: and if thou redeem him not, then shalt thou break his neck. All the firstborn of thy sons thou shalt redeem. And none shall appear before me empty.
This command was given during Moses' second forty days and nights alone in the mount, when he was miraculously sustained without ordinary support. At God's command he had prepared two tables of stone, and the Lord wrote upon them the words of the covenant, the Ten Commandments. After that long communion Moses came down with his face reflecting the glory of the divine presence with a dazzling light. That brightness made Aaron and the people afraid to come near him. In compassion for their fear he afterward put a veil over his face when he returned to the camp from communion with God.
The command requires the firstling of a donkey to be redeemed with a lamb, and all the firstborn sons to be redeemed, and it requires that no one appear before God empty-handed. It therefore teaches that everyone is to bring an offering according to his means, and poverty excuses no one from giving. Those who are unable to give because of sickness or misfortune are to be helped by the wealthy, so that even they can still bring a humble offering.