Deuteronomy 6:13

13 Fear the LORD your God, serve him only and take your oaths in his name.

Read Deuteronomy 6:13 Using Other Translations

Thou shalt fear the LORD thy God, and serve him, and shalt swear by his name.
It is the LORD your God you shall fear. Him you shall serve and by his name you shall swear.
You must fear the LORD your God and serve him. When you take an oath, you must use only his name.