Does the monster have to be on the field in order to use it in a tribute summon? I hope I haven't been living under rocks for the past few years of playing at home where I used cards in my hand for tributes.

tributes must always be from the field, unless otherwise stated (such ritual summons, or manticore of darkness's effect)

Tributes for anything must always be from your own side of the field unless an effect specifies otherwise...

Ritual Summons specifically stating "from your hand or field".

Soul Exchange, Lava Golem, Alien Skull, etc allowing you to Tribute from your opponent's side of the field.

Thanks guys.

This was a serious hit to my "ability" to follow instructions.