Updated for Ubuntu 18.04
Click on the icons on top right corner and select Screwdriver & Wrench icon at the bottom of the drop-down menu. Then select the Devices from the left panel. Then select the Keyboard from the left panel. This will bring you to the table of keyboard shortcuts.
Scroll down to the Typing section and find two entries:
- Switch to next input source Super+Space
- Switch to previous input source Shift+Super+Space
Select Switch to next input source and follow the instructions on the screen and press the keyboard shortcut you want to use such as Ctrl+Space.
Select Switch to previous input source and follow the instructions on the screen and press the keyboard shortcut you want to use such as Shift+Ctrl+Space.
Original Answer
Click on the power/Gear icon on top right corner and select System Settings... Then select the Text Entry icon under Personal. Make sure Persian is listed. If not, add it by clicking on the + sign.
I find it hard to use any combination with Alt as it brings up the HUD. To change this return to Text Entry as before and click on the space under Switch to next source using: where it says Alt+Shift and enter another combination such as Cntrl+Space.
Next click on the space under Switch to previous source using and enter another combination such as Shift+Cntrl+Space.
Hope this helps