Sn 5.14 Posala manava puccha
Posala-manava-puccha: Posala's Questions
translated from the Pali by
To one who reveals the past unperturbed, his doubts cut through who has gone to the beyond of all phenomena, I've come with a question. I ask the Sakyan about the knowledge  of one devoid of perception of forms, who has abandoned all the body, every body, who sees, within & without, 'There is nothing': How is he to be led further on?
The Tathagata, knowing directly all stations of consciousness,  knows for one stationed in them release & the steps leading there. Knowing directly the origin of nothingness to be the fetter of delight, one then sees there clearly. That's his genuine knowledge the brahman who has lived to fulfillment.
1. Posala's question concerning the knowledge of the person in the dimension of nothingness has a double meaning: He is asking about the Buddha's knowledge about that person, and also what a person in that dimension of attainment should do to develop his/her knowledge even further. The Buddha's answer deals with the question in both its senses.
2. On the seven stations of consciousness, see DN 15. The dimension of nothingness, discussed in this dialogue, is the seventh and most refined.