釋禪波羅蜜次第法門 3.27

3.27

若繫心臍下，臍是氣海，亦曰中宮，繫心在臍，能除眾病，或時內見三十六物，發特勝等禪。

If one ties the mind below the navel, the navel is the ocean of qi, also called the central palace; tying the mind to the navel can eliminate various illnesses, or at times one inwardly sees the thirty-six components [of the body], arousing dhyānas such as the sublime practices.