De Anima
Tertullian
For instance, the Nasamones consult private oracles by frequent and lengthened visits to the sepulchres of their relatives, as one may find in Heraclides, or Nymphodorus, or Herodotus; and the Celts, for the game purpose, stay away all night at the tombs of their brave chieftains, as Nicander affirms.
Tertullian, De Anima. translated by Peter Holmes, D.D.