How to record camera with sound from external source on an iPhone 7?
There are a whole heap of external microphones you can use with the iPhone 7.

Use the included adapter

Firstly, the most basic option, is to simply use the Lightning Port > 3.5mm adapter that comes with the phone. This adapter supports Tip Run Ring Sleeve (TRRS), so you could just plug in an external microphone into the iPhone 7 that way (i.e. an external microphone that supports the TRRS plug).

Lightning port microphones

However, if you're looking for a more dedicated solution without an extra adapter in the way, you do have a number of options available if you want to connect directly via the Lightning port. Below are some options for you (ranging in quality, price and features):


Sennheiser ClipMic Digital 
iRig Pro
iRig Mic HD
iRig Field
iRig Pro Duo
iRig Mic Studio
Rode i-XLR
Shure MV88
Zoom iQ5
Zoom iQ6


Disclaimer: I am not affiliated with any of the above products.