"He passed over the houses of the Israelites in Egypt when He struck the Egyptians, and He spared us."

Exodus Chapter 12

"And God said to Moses and Aaron: This month shall be the head month for you; it shall be the first month of the year... and you shall eat the sacrificial meat during this night, roasted over fire. You shall eat it with matza and bitter herbs... This day must be one that you will remember. 

You must keep it as a festival to God for all generations; it is a law for all time that you shall celebrate it... Eat matza for seven days... You must keep this as a law, for you and your children forever. 

When you come to the Land that God will give you, as He promised you, you must also keep this service. Your children may then ask you, 'what is this service to you?' You must answer, 'It is the Passover service to God. He passed over the houses of the Israelites in Egypt when He struck the Egyptians, and He spared us."