Jamieson, Fausset and Brown on Exodus 12:3

3. Speak ye unto all the congregation of Israel--The recent events had prepared the Israelitish people for a crisis in their affairs, and they seem to have yielded implicit obedience at this time to Moses. It is observable that, amid all the hurry and bustle of such a departure, their serious attention was to be given to a solemn act of religion.

a lamb for an house--a kid might be taken (Ex 12:5). The service was to be a domestic one, for the deliverance was to be from an evil threatened to every house in Egypt.