佛說觀佛三昧海經 1.66

1.66

佛告父王：「佛滅度後，若四部眾，欲觀如來苦行時白毫相者，當作是觀。如此觀者是名正觀，若異觀者名為邪觀。」

The Buddha said to his father the king: "After the Buddha's parinirvāṇa, if members of the fourfold assembly wish to contemplate the mark of the Tathāgata's white curl at the time of ascetic practice, they should contemplate in this way. Contemplating in this way is called correct contemplation; contemplating otherwise is called false contemplation."