1… "Here, friends, while I was alone in seclusion, a reflection arose in my mind thus: ‘It is said, "the signless concentration of mind, the signless concentration of mind." What now is the signless concentration of mind?’ [n.280] Animitta cetosamādhi. Spk: This refers to insight concentration (vipassanāsamādhi), which occurs when one has abandoned the sign of permanence, etc.
The "signless concentration of mind" is not defined further in the Nikāyas, but its placement after the eighth formless attainment suggests it is a samādhi qualitatively different from those attained in samatha meditation. Below, it occurs in the explanation of the "signless liberation of mind" (animittā cetovimutti, at SN41.7; IV 297,3–6). At SN43.4, the signless concentration (animitta samādhi) is called the path leading to the unconditioned. For a wide-ranging overview of the signless meditation, see Harvey, "Signless Meditation in Pali Buddhism." See too below nn. 312, 368.

"Then, friends, it occurred to me: ‘Here, sn.iv.269 by nonattention to all signs, a bhikkhu enters and dwells in the signless concentration of mind. This is called the signless concentration of mind.’

2"Then, friends, by nonattention to all signs, I entered and dwelt in the signless concentration of mind. While I dwelt therein my consciousness followed along with signs. [n.281] Nimittānusāri viññāṇaṃ hoti. Spk: This occurred while his insight knowledge was flowing along sharply and strongly as he dwelt in insight concentration. Just as, when a man is cutting down a tree with a sharp axe, if he constantly inspects the blade he doesn’t accomplish the function of cutting down the tree, so the elder developed a liking (nikanti) for insight and thus did not accomplish its function.

"Then, friends, the Blessed One came to me by means of spiritual power and said this: ‘Moggallana, Moggallana, do not be negligent, brahmin, regarding the signless concentration of mind. Steady your mind in the signless concentration of mind, unify your mind in the signless concentration of mind, concentrate your mind in the signless concentration of mind.’ Then, on a later occasion, by nonattention to all signs, I entered and dwelt in the signless concentration of mind.

"If, friends, one speaking rightly could say of anyone: ‘He is a disciple who attained to greatness of direct knowledge with the assistance of the Teacher,’ it is of me that one could rightly say this."

1"‘Animitto cetosamādhi, animitto cetosamādhī’ti vuccati. Katamo nu kho animitto cetosamādhīti?

Tassa mayhaṁ, āvuso, etadahosi: ‘idha bhikkhu sabbanimittānaṁ amanasikārā animittaṁ cetosamādhiṁ upasampajja vihārati. Ayaṁ vuccati animitto cetosamādhī’ti.

So khvāhaṁ, āvuso, sabbanimittānaṁ amanasikārā animittaṁ cetosamādhiṁ upasampajja viharāmi. Tassa mayhaṁ, āvuso, iminā vihārena vihārato nimittānusāri viññāṇaṁ hoti.

2Atha kho maṁ, āvuso, bhagavā iddhiyā upasaṅkamitvā etadavoca: ‘moggallāna, moggallāna. Mā, brāhmaṇa, animittaṁ cetosamādhiṁ pamādo, animitte cetosamādhismiṁ cittaṁ saṇṭhapehi, animitte cetosamādhismiṁ cittaṁ ekodiṁ karohi, animitte cetosamādhismiṁ cittaṁ samādahā’ti. So khvāhaṁ, āvuso, aparena samayena sabbanimittānaṁ amanasikārā animittaṁ cetosamādhiṁ upasampajja vihāsiṁ.

Yañhi taṁ, āvuso, sammā vadamāno vadeyya: ‘satthārānuggahito sāvako mahābhiññataṁ patto’ti, mamaṁ taṁ sammā vadamāno vadeyya: ‘satthārānuggahito sāvako mahābhiññataṁ patto’"ti.

Navamaṁ.