Some time ago I visited a friend, and had him enter the Wi-Fi password in my phone. Next time I visited I had my PC with me, the PC of course was not able to connect to the network without the password. Being too lazy I was trying to figure out if there is any way to recall the password saved on my phone; turns out, there is.

Note: On Android, this requires root access.

1. Open a file explorer¹ which has the availability to browse as root. I personally use FX File Explorer with root addon.

2. Open the following file in a text reader:

   /data/misc/wifi/wpa_supplicant.conf
   

3. Scroll down until you find the network and the PSK you are looking for.

Easy as that.

Update 1 (2017-12-26): To find saved PSK on Windows 10. Update 2 (2017-01-18): Command for using adb:

adb shell "su -c 'cat /data/misc/wifi/wpa_supplicant.conf'"

Source: XDA

Links

Play Store - FX File Explorer Play Store - FX File Explorer Root Addon XDA Developers - Any way to retrieve stored wifi password from Android device without root and ADB?? Windows 10: Find Wi Fi Password