坐禪三昧經 2.6

2.6

此覺、觀二事，亂禪定心，如水澄靜，波蕩則濁，行者如是，內已一心，覺觀所惱，如極得息，如睡得安。是時次第無覺無觀，生清淨定，內淨喜樂，得入二禪。

These two things, initial thought and sustained thought, disturb the mind in dhyāna concentration. Just as clear water is muddied when agitated by waves, so the practitioner, having already attained one-pointedness within, is distressed by initial and sustained thought; [eliminating them is] like gaining rest when exhausted, like finding peace in sleep. At that time, in sequence, free of initial thought and sustained thought, pure concentration is born, with internal clarity, joy, and bliss, and he attains entry into the second dhyāna.