Ring of the Archmagi (4)

A powerful artifact with a sliver of a wondrous gem inset. Increases Strength, Agility and Intelligence of the Hero by 3 and gives nearby friendly units a bonus to mana regeneration.

Source
Kael and Lady Vashj can find one in The Dungeons of Dalaran, chapter 3 of the Frozen Throne alliance campaign, Curse of the Blood Elves.