This creation myth dates back to AD&D 1st edition book Deities & Demigods, which first defined Corellon Larethian:
The god is also mighty in battle, and said to have personally banished such demons as Lolth from the sunlit Upperworld. Elven lore states that the race of elves sprang from the drops of blood Corellon shed in this epic battle.
In the AD&D 2nd edition Monster Mythology (1992), it is Gruumsh who spilled Corellon's blood:
The birth of the elves, people of song and dance and quiet and still places, is believed by them to have originated in the terrible battle Corellon fought with Gruumsh One-Eye, First Power of the orcs. The elves were born from Corellon's blood, and they do not forget this.