De Iside et Osiride - Plutarch

The ibis, which kills the deadly creeping things, was the first to teach men the use of medicinal purgations when they observed her employing clusters and being purged by herself. The most strict of the priests take their lustral water for purification from a place where the ibis has drunk for she does not drink water if it is unwholesome or tainted, nor will she approach it. By the spreading of her feet, in their relation to each other and to her bill, she makes an equilateral triangle. Moreover the variety and combination of her black feathers with her white picture the moon in its first quarter - Plutarch, De Iside et Osiride