Atmega32u4 external 8MHz clock
See §28.2 in Atmel-7766-8-bit-AVR-ATmega16U4-32U4_Datasheet.pdf, which explains usage of various fuse bits.

Also see engbedded.com/fusecalc, which lets you easily figure out fuse settings.  Fusecalc also tells you the strings to use as part of avrdude command strings.

Once you know the fuse settings, send them to your device via a command like  

avrdude -p m32u4 -c usbasp -u -U lfuse:w:0xEF:m -U hfuse:w:0xD5:m -U efuse:w:0x07:m


or like 

avrdude -p m32u4 -P $port -c arduino -u -U lfuse:w:0xEF:m -U hfuse:w:0xD5:m -U efuse:w:0x07:m


where $port represents a port name like /dev/ttyUSB0.

Note, it makes sense to read the fuses first, and put the readings into Fusecalc to see what they correspond to.  You can read the fuses via a command like

avrdude -p m32u4 -P $port -c arduino -U hfuse:r:-:i