Every firstling of an ass thou shalt redeem with a lamb - Or a kid, as in the margin. In Numbers 18:15, it is said: "The first-born of man shalt thou surely redeem; and the firstling of an unclean beast shalt thou redeem." Hence we may infer that ass is put here for any unclean beast, or for unclean beasts in general. The lamb was to be given to the Lord, that is, to his priest, Numbers 18:8, Numbers 18:15. And then the owner of the ass might use it for his own service, which without this redemption he could not do; see Deuteronomy 15:19.

The first-born of man - shalt thou redeem - This was done by giving to the priests five standard shekels, or shekels of the sanctuary, every shekel weighing twenty gerahs. What the gerah was, see Clarke on Genesis 20:16 (note). And for the shekel, see Clarke on Genesis 20:16 (note).

It may be necessary to observe here that the Hebrew doctors teach, that if a father had neglected or refused thus to redeem his first-born, the son himself was obliged to do it when he came of age. As this redeeming of the first-born was instituted in consequence of sparing the first-born of the Israelites, when the first-born both of man and beast among the Egyptians was destroyed, on this ground all the first-born were the Lord's, and should have been employed in his service; but he permitted the first-born of a useful unclean animal to be redeemed by a clean animal of much less value. And he chose the tribe of Levi in place of all the first-born of the tribes in general; and the five shekels were ordered to be paid in lieu of such first-born sons as were liable to serve in the sanctuary, and the money was applied to the support of the priests and Levites. See this subject at large in Numbers 3:12, Numbers 3:13, Numbers 3:41, Numbers 3:43, Numbers 3:45, Numbers 3:47-51.

thy son

Exodus 12:26 And it shall come to pass, when your children shall say to you, What mean you by this service?

Deuteronomy 6:20-24 And when your son asks you in time to come, saying, What mean the testimonies, and the statutes, and the judgments...

Joshua 4:6,21-24 That this may be a sign among you, that when your children ask their fathers in time to come, saying, What mean you by these stones...

Psalm 145:4 One generation shall praise your works to another, and shall declare your mighty acts.

in time to come. Heb. tomorrow

Exodus 12:26 And it shall come to pass, when your children shall say to you, What mean you by this service?

Genesis 30:33 So shall my righteousness answer for me in time to come, when it shall come for my hire before your face...

Deuteronomy 6:20 And when your son asks you in time to come, saying, What mean the testimonies, and the statutes, and the judgments...

Joshua 4:6 That this may be a sign among you, that when your children ask their fathers in time to come, saying, What mean you by these stones?

Joshua 22:24 And if we have not rather done it for fear of this thing, saying, In time to come your children might speak to our children, saying...

by strength

Exodus 13:3 And Moses said to the people, Remember this day, in which you came out from Egypt, out of the house of bondage...

The Personality of Power.
A Personally Conducted Journey. Everyone enjoys the pleasure of travel; but nearly all shrink back from its tiresomeness and drudgery. The transportation companies are constantly scheming to overcome this disagreeable side for both pleasure and business travel. One of the popular ways of pleasure travel of late is by means of personally conducted tours. A party is formed, often by the railroad company, and is accompanied by a special agent to attend to all the business matters of the trip. A variation …
S.D. Gordon—Quiet Talks on Power

Jesus' Last Public Discourse. Denunciation of Scribes and Pharisees.
(in the Court of the Temple. Tuesday, April 4, a.d. 30.) ^A Matt. XXIII. 1-39; ^B Mark XII. 38-40; ^C Luke XX. 45-47. ^a 1 Then spake Jesus ^b 38 And in his teaching ^c in the hearing of all the people he said unto ^a the multitudes, and to his disciples [he spoke in the most public manner], 2 saying, ^c 46 Beware of the scribes, ^a The scribes and the Pharisees sit on Moses' seat: 3 all things whatsoever they bid you, these do and observe: but do not ye after their works: for they say, and do not. …
J. W. McGarvey—The Four-Fold Gospel

The Old Testament Canon from Its Beginning to Its Close.
The first important part of the Old Testament put together as a whole was the Pentateuch, or rather, the five books of Moses and Joshua. This was preceded by smaller documents, which one or more redactors embodied in it. The earliest things committed to writing were probably the ten words proceeding from Moses himself, afterwards enlarged into the ten commandments which exist at present in two recensions (Exod. xx., Deut. v.) It is true that we have the oldest form of the decalogue from the Jehovist …
Samuel Davidson—The Canon of the Bible