Spellcasting drain resistance test, SR3.183. "If the force of the spell is greater than the caster's Magic Attribute, the Drain causes physical damage."

Conjuring drain resistance test, SR3.188 top left corner: "If the spirit's Force is greater than the summoner's Magic Attribute, the Drain causes physical damage."

Magic Increase, MitS.58: "Initiation increases the initiate's Magic Attribute. For each grade of initiation, the character gains 1 point of Magic..."

Also related:
Power Focus, SR3.190: A power focus increases the owner's Magic Attribute by its own Force for purposes of determining whether Drain does Stun or Physical.

Ally Spirit Aid Power, MitS.108: When using the "Aid Power", the ally acts as a power focus, adding its Force to its master's Magic for purposes of Drain.

Limited Spells, SR3.180: Taking a limit (Exclusive or Fetish) on a spell when learning it may either reduce the Force of a spell for purposes of Drain, or reduce the Karma cost and difficulty to learn the spell.

Astral Spellcasting, SR3.182: "Drain from spells cast while astrally projecting always causes physical damage, regardless of the Force of the spell."