Deuteronomy 11:13

And it shall come to pass, if you shall hearken diligently
unto my commandments which I command you this day
In the name, and by the authority of the Lord, the only lawgiver:

to love the Lord your God, and to serve him with all your heart, and
with all your soul;
see ( Deuteronomy 10:12 ) . Jarchi interprets this of prayer; but it is not to be restrained to that only, but includes the whole service of God, in all the parts of it, performed from a principle of love to him, and in sincerity and truth.