In Robert de Boron’s Joseph d’Arimathie, Peter, a follower of Joseph of Arimathea, is instructed to journey to the Vale of Avaron to await the coming of Alain’s son, probably Perceval.
Avaron almost certainly is meant to be Avalon, which would make this romance the first reference of Avalon as a valley rather than an island.
Source
Joseph d’Arimathie | Robert de Boron, 1191–1202