The key combination which collapses the ribbon is:

[Ctrl]+[F1]

The key combination to collapse the ribbon in Microsoft Office applications is Ctrl+F1. To collapse the ribbon, press and hold the Ctrl key and then press the F1 key. To expand the ribbon again, press and hold the Ctrl key and then press the F1 key again. You can also collapse the ribbon by clicking on the Collapse the Ribbon button in the bottom right corner of the ribbon. Collapsing the ribbon can be useful if you need more screen space to work on your document or spreadsheet. It can also be helpful if you find the ribbon to be distracting.