佛說觀佛三昧海經 2.27

2.27

「如我滅後，欲觀如來降伏魔時白毛相者，當作此觀，如是觀者名為正觀，若異觀者名為邪觀」。

"After my parinirvāṇa, those who wish to contemplate the mark of the white curl when the Tathāgata subdued the demon should practice this contemplation. To contemplate in this way is called correct contemplation; to contemplate otherwise is called incorrect contemplation."