The second commandment

4 "You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth.

 5 You shall not bow down to them or serve them, for I the LORD your God am a jealous God, visiting the iniquity of the fathers on the children to the third and the fourth generation of those who hate me,

 6 but showing steadfast love to thousands of those who love me and keep my commandments.

-- Exodus 20.4-6, ESV 

I know some people consider this to be part of the first commandment, but in all honesty I think they're wrong.

In my experience, people tend to focus on verse five's description of God as jealous and as someone who punishes children for what their fathers did. They never seem to get to the promise of unfailing love in the next verse.