Soulfire Grand Master by paying ({2} magic symbol) ({UR} magic symbol) ({UR} magic symbol) , causing you to put the next instant or sorcery card you cast from your hand this turn into your hand as it resolves.
Cast
Mana Geyser by paying ({3} magic symbol) ({R} magic symbol) ({R} magic symbol) , adding ({R} magic symbol) for each tapped land opponents control and returning
Mana Geyser to your hand.
Repeat.
Notes
If opponents control ten or more tapped lands, this combo nets infinite red mana.