Exodus 13:2

Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine.

Sanctity unto me all the first-born. To sanctify means to consecrate, to set apart from a common to a sacred use. The foundation of this duty rested on the fact, that the Israelites having had their first-born preserved by a distinguishing act of grace from the general destruction that overtook the families of the Egyptians, were bound, in token of gratitude, to consider them as the Lord's special property (cf. Hebrews 12:23). By that deliverance Israel, God's son, His first-born, was brought into a new and special relation, which gave it a national existence to be distinguished by extraordinary religious privileges; and it was proper, therefore, in accordance with this sonship, that the first-born, as representatives of all the rest, should be sanctified to the Lord. Hence, the duty was specified to the leader on the earliest possible occasion; and from the place which it occupies in the record, the enactment seems to have been made at Succoth.