Exodus 12:3-4

3Tell all the congregation of Israel, ‘On the tenth [day] of this month they are to take a lamb or young goat for themselves, according to [the size of] the household of which he is the father, a lamb or young goat for each household.

4Now if the household is too small for a lamb [to be consumed], let him and his next door neighbor take one according to the number of people [in the households]; according to what each man can eat, you are to divide the lamb.