Need help with a code

I have for 16 years always been told that a water heater must have a min of 18" of ridgid pipe connected to it before you can tie into pex or polly. I have been burried in the 2015 IPC for a couple of hours because an apprentice at my shop inquired about this and I told him I would look into the code book and get back to him. I just cant find where it states that you can't tie pex directly into a water hater and that you have to have 18" of ridgid pipe. I have always used copper or brass on the top of my what heater for this reason. Someone please help me here I am feeling stupid as I am the senior plumber at my shop.

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