Before I answer this question, I have to make one thing clear: DON'T EVER DO THIS!
Verizon will still replace your phone under warranty with an unlocked bootloader as long as there is no physical damage. Motorola will do the same. Even if they do decide to get picky about bootloaders, re-locking your bootloader does not erase all traces of it ever being unlocked. Your bootloader will show status code 2, which indicates that it has been unlocked and re-locked again. If you lock it before you sell it to someone else, and they try to unlock it again, the phone will be ruined. There is absolutely no reason to do this.
With that out of the way, the command is "fastboot oem lock"
Don't ever use it.