The earliest report of a monster in the vicinity of Loch Ness comes from the biography of Saint Columba by Adomnán. Writing a century after the actual event, Adomán describes the Irish monk’s interaction with the beast not in the lake itself but the River Ness which feeds into the lake.
Columba, after hearing how a local man was mauled to death by a water-beast, sends one of his companions to swim across the river. About halfway across the river, the man is pursued by the beast, but Columba makes the sign of the cross and commands the monster to turn around, allowing his associate to continue swimming.
While not much stock is given to the report, as water-beast tales were often used in the exploits of Saints, taken in context with the many sightings, Nessie proponents claim that it adds validity to the monster’s existence.