Numbers 9:2

And the LORD spake unto Moses in the wilderness of Sinai, in the first month of the second year after they were come out of the land of Egypt, saying,

In the fourteenth day of this month, at even, ye shall keep it in his appointed season: according to all the rites of it, and according to all the ceremonies thereof, shall ye keep it.

And Moses spake unto the children of Israel, that they should keep the passover.

And they kept the passover on the fourteenth day of the first month at even in the wilderness of Sinai: according to all that the LORD commanded Moses, so did the children of Israel.