Visuddhimagga X-33

Taṃ viññāṇaṃ amanasikaritvā ‘‘natthi natthī’’ti vā, ‘‘suññaṃ suñña’’nti vā, ‘‘vivittaṃ vivitta’’nti vā punappunaṃ āvajjitabbaṃ, manasikātabbaṃ, paccavekkhitabbaṃ, takkāhataṃ vitakkāhataṃ kātabbaṃ.

即不作意于识，却对「无，无」，或「空，空」，或「远离，远离」的数数念虑、作意、观察与思惟。

Ñ(X,33): Without giving [further] attention to that consciousness, he should [now] advert again and again in this way, 'there is not, there is not', or 'void, void', or 'secluded, secluded', and give his attention to it, review it, and strike at it with thought and applied thought.