坐禪三昧經 1.14

1.14

If the mind becomes extremely fatigued, let mindfulness dwell on its focal object, abandoning outer things and holding to what is retained. For example, just as a monkey is tied to a pillar and stops to rest only when exhausted: the focal object is like the pillar, mindfulness is like the tether, and the mind is compared to the monkey. It is also like a wet nurse who constantly watches over an infant so that it does not fall. The practitioner's observation of the mind is also like this, gradually restraining the mind to make it dwell upon the focal object.