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.

https://aonprd.com/Rules.aspx?ID=361

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

Pathfinder has that too, so it presumably carried through 3.5e. It’s why wizards don’t wear armor, and only applies to arcane casters, and classes that are meant to wear some armor like bards get exemptions for the tiers of armor they’re meant to wear.