It is a combination of buttons, but it's been quite a while for me, try these:
Press the power button then tap the down volume button.
-or-
Press the power button and tap the volume up button.
-or-
Press the power button and then tap the home button.
I am just so used to running a custom ROM on my devices where it is usually an option that you can enable in other areas of the system interface.
good luck