佛說觀佛三昧海經 7.39

7.39

Furthermore, there were transformation Buddhas who taught the śrāvakas the counting of breaths (ānāpāna), radiant light upon white bones and white bones upon radiant light, the thought of mental purity and the thought of mental impurity, the thought of the arising of the defilements and the thought of the cessation of the defilements, the thought of severing the defilement branches and the thought of slaying the defilement roots. Such thoughts were as numerous as nine hundred billion dust motes, as expounded in the counting of breaths; these are called the teachings for śrāvakas.