Recommendation from a trusted 3rd party.

Bill Gurley has a great line about this:

"I use LinkedIn like this:

If Person A reaches out to me and there is a Person B that is a common connection between A and myself, I want to be able to call Person B and have 100% confidence in their evaluation. That's the bar I set to connect with someone on LinkedIn."

From:

https://www.youtube.com/watch?v=xmYekD6-PZ8