金剛經纂要刊定記 2.178

2.178

三、觀心無常者。心即緣慮生滅之心。謂心心念念前滅後生相續不絕，如水流注。故經中說一念中有九十剎那，一剎那中九百生滅。

Third, contemplating mind as impermanent: The mind is just the mind of cognizing conditions, arising and ceasing. That is to say, from thought-moment to thought-moment, preceding ones cease and succeeding ones arise, continuous without interruption, like a flowing stream of water. Therefore, a sūtra states that in a single thought-moment there are ninety kṣaṇas, and in a single kṣaṇa there are nine hundred arisings and ceasings.