Exodus 12:6
6 and you shall keep it until the ifourteenth day of this month, when the whole assembly of the congregation of Israel shall kill their lambs at twilight.1
Numbers 9:5
5 And they kept the Passover in the first month, on the fourteenth day of the month, at twilight, in the wilderness of Sinai; according to all that the Lord commanded Moses, so the people of Israel did.