Necklace of Form Stability

This item give a +3 Saving Throw bonus versus Polymorph.

In Baldur's Gate: Enhanced Edition it is sold by Xithiss Tier 1 store in the Black Pits.

In Baldur's Gate II, this can be found in Sewers in Mekrath's Lair.

In-game description
"Necklace of Form Stability: Dydaar'a of the Altered was a witch of great temper who rewarded the slightest insult with harsh punishment. Often this meant several weeks transformed into some form of wild beast, which of course earned her many enemies. It is recorded that she once fought a Mage named Maxell who had previously suffered such a fate. For his return he had prepared this necklace, designed to prevent her changing his form, but he could not shield those around him as well. Apparently an impromptu army of farmers-turned-bears eventually overwhelmed him."