Casting an Arcane Spell in Armor: A character who casts an arcane spell while wearing armor must usually make an arcane spell failure check. The number in the Arcane Spell Failure Chance column on Table 6–6 is the percentage chance that the spell fails and is ruined. If the spell lacks a somatic component, however, it can be cast with no chance of arcane spell failure.

aonprd.com/Rules.aspx?ID=361

It was a rule in Pathfinder, so presumably it was a rule in 3e.