Scroll of Regeneration

Regenerates the hit points of all friendly non-mechanical units an an area around your Hero by 225 over 45 seconds when used.

This scroll is inscribed on crisp, clean parchment that stays a brilliant white even if exposed to dirt, grime, or mud. The words inscribed thereon are magical, but easily understood, and are written in a circle that rotates when read, allowing the inscribtions to be easily repeated.

Source
These can be bought from Arcane Vaults in Frozen Throne.

Malfurion, Tyrande and Maiev can find one in Balancing the Scales, chapter 5 of the Frozen Throne sentinels campaign, Terror of the Tides.

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