From the Super Mario Wiki
A Magic Pendant is a small, yet powerful piece of jewelry, two of which are owned by the Mouth of the River.
In The Super Mario Bros. Super Show! episode "Rolling Down the River" the Mouth of the River, wanting to aid Mario and Luigi in their quest to rescue Toad and Princess Toadstool, gave them her two Magic Pendants. Once Mario and Luigi put these Magic Pendants on, they were transformed into Super Mario and Super Luigi.
Unfortunately, Mario and Luigi, after beating Captain Koopa, lost their Magic Pendants and their superpowers when the Magic Pendants were swiped from around their neck by Captain Koopa; it is unknown what happened to the Magic Pendants afterwards.