Numbers 9:5

And they kept the passover on the fourteenth, day of the first
month at even in the wilderness of Sinai
No mention is made of keeping the feast of unleavened bread seven days, only of the passover, which indeed was only enjoined at this time, though the feast of unleavened bread used to follow it, and did in later times; but perhaps it would not have been an easy matter to have got the flour to make it of, sufficient for so large a body of people, for seven days together in the wilderness; though they might be able to furnish themselves with what was enough for one meal from the neighbouring countries, and especially from Midian, where Jethro, Moses's father, lived, and which was not very far from Sinai, where the Israelites now were:

according to all that the Lord commanded Moses, so did the children of
Israel;
which is observed to their honour; though Jarchi gives this as a reason why this book does not begin with this account, as the order of things seems to require, because it was to the reproach of the Israelites, that all the forty years they were in the wilderness they kept but this passover only; the reason of which was, because of the omission of circumcision during that time, through the inconveniences of travelling, and the danger of circumcision in it, without which their children could not eat of the passover, ( Exodus 12:48 ) .