We are playing a Shadowrun 5 campaign. When a mage casts a spell, they have to roll a Drain resistance check. The GM is requiring multiple drain checks (one for each turn the mage sustains the spell) in case of sustained spells. But it seems to me like such a spell does not requires multiples checks by the rules, just one right after casting the spell. Which one of these two options is supported by the rules?
You resist drain when you roll the spellcasting test, not when sustaining.
SR5 p.278 under Drain:
The base Drain Value for Sorcery or Enchanting tests is listed with the appropriate spell, ritual or preparation description.
You only test when you cast the spell. You don't roll a sorcery test to sustain a spell.
SR5 p.282 under Resist drain:
After casting a spell you must resist drain using the dice pool for Drain Resistance according to your tradition.
This is from the detailed step by step guide on how to cast spells. Resisting drain is done after casting the spell.
SR5 p.282 under Determine ongoing effects:
If you sustain the spell you take a -2 dice pool penalty to all tests while it is sustained.
This step does not mention any additional drain resists, nor any additional sorcery tests. Instead there is a -2 dice pool penalty for sustaining spells. Since this is a detailed step by step guide any additional tests would have been mentioned here.