Adam Clarke's Commentary on the Bible

Verse 3 (Matthew 6:3)

Let not thy left hand know - In many cases, works of charity must be hidden from even our nearest relatives, who, if they knew, would hinder us from doing what God has given us power and inclination to perform. We must go even farther; and conceal them as far as is possible from ourselves, by not thinking of them, or eyeing them with complacency. They are given to God, and should be hidden in Him.

- Adam Clarke's Commentary on the Bible