佛說觀佛三昧海經 1.44

1.44

「今為父王說生頂相，若有聞者應當思惟佛勝頂相，其相光明，如三千界大地微塵，不可具說。後世眾生若聞是語，思是相者心無悔恨，如見世尊頂勝相光，閉目得見，以心想力了了分明如佛在世。雖觀是相不得眾多，從一事起復想一事，想一事已復想一事，逆順反覆經十六反，如是心相極令明利。然後住心繫念一處，如是漸漸舉舌向腭令舌政住，經二七日，然後身心可得安隱。復當繫心還觀佛頂，觀佛頂法先隨毛孔入。」

"Now for my father the king I expound the mark of the crown at birth. If anyone hears it, they should reflect on the Buddha's supreme mark of the crown; the light of that mark is like the fine dust of the great earth in the three-thousandfold world, impossible to describe fully. If sentient beings of future generations hear these words and contemplate this mark without regret in their minds, as if seeing the light of the Bhagavat's supreme mark of the crown, they will be able to see it with eyes closed; through the power of mental visualization it will become distinctly clear, just as when the Buddha was in the world. Although contemplating this mark, one should not take on too many at once: begin with one feature and then visualize another feature; having visualized that feature, again visualize another feature, reversing and repeating in forward and reverse sequence through sixteen turns; in this way cause the mental image to become exceedingly clear and sharp. Thereafter, stabilize the mind and focus thought upon a single point. In this manner, gradually raise the tongue toward the palate, keeping the tongue straight and steady; after two times seven days, body and mind may attain tranquility. Then one should again focus the mind to return to contemplating the Buddha's crown; the method for contemplating the Buddha's crown is first to enter through the pores of the skin."