Summer grasses / All that remains / Of soldiers' dreams. - Basho.
Sean has already answered it correctly, but in case you're looking for the rules section, it's 6.1 of the Rules (ver.2.1)
6.1 FIRE RESTRICTIONS AND EFFECTS
A unit must pass a MC before firing (Exception: If using
Assault Fire, the unit already passed a MC before moving and
does not need to pass another one before firing).
Units may NOT fire if in the same hex as an enemy unit.
Units may not fire into a hex containing friendly Infantry or
Guns, but may fire through a hex containing enemy or friendly
units (or both). Each unit must execute its fire attack
individually. No units may combine fire attacks.
Relevant sections made bold for emphasis.