摩訶止觀 4.158

4.158

若掉散者，應用數息。何以故？此蓋甚利，來時不覺，于久始知。今用數息，若數不成，或時中忘，即知已去，覺已更數。數相成就則覺觀被伏；若不治之，終身被蓋。

If one is restless and scattered, one should use counting the breath. Why? This hindrance is extremely sharp; when it comes, one does not notice, realizing it only after a long time. Now, using counting the breath: if the count is not accomplished, or if one forgets midway, one immediately knows that [the mind] has wandered off; realizing it, one counts again. When the marks of counting are accomplished, initial and sustained thought are subdued; if one does not cure it, one will be hindered throughout one's life.