佛說觀佛三昧海經 1.49

1.49

"If there are monks, nuns, upāsakas, and upāsikās who wish to contemplate the Buddha's hair, they must practice this contemplation; they must not contemplate otherwise. If they contemplate otherwise, it is called erroneous contemplation, it is called madness and confusion, it is called loss of mind, it is called wrong view, it is called an inverted mind; even if they were to attain absorption, it would be impossible. Thus, father the king, such is the factual reality of the Buddha's true mark of the hair.