Speak/talk of the devil
Definition 1 of 1
Pro Tip 1/3Coincidence Highlight
Use it to emphasize a coincidence when someone appears just as they are mentioned. 
We were just discussing who should lead the project, and speak of the devil, Sarah walks in.
Pro Tip 2/3Informal Usage
It's a casual expression, more common in informal conversations than formal settings. 
We were just laughing about old times and, speak of the devil, our old coach appears.
Pro Tip 3/3Not Insulting
Despite 'devil', it's not used to offend but can be a playful way to acknowledge someone's arrival. 
You missed Pete's hilarious joke, and oh, speak of the devil, here's Pete now!