Tertullian, On the Soul.

And it is often alleged because of nightmare dreams that the dead truly appear, for the Nasamones [Libyan nomads] receive special oracles by staying at the tombs of their parents … The Celts also for the same reason spend the night near the tombs of their famous men, as Nicander affirms.

[Quoted in Fionntán de Brun, in ‘Temporality and Revivalism’, UU Research Series, April 2011.]