Animitta, in suttas with parallels
From distorted perception your mind is on fire. Shun the sign of the beautiful accompanied by lust. See mental fabrications as other, as stress, & not-self. Extinguish your great lust. Don't keep burning again & again. Develop the mind - well-centered & one - in the foul, through the foul. Have your mindfulness immersed in the body. Be one who pursues disenchantment. Develop the signless. Cast out conceit. Then, from breaking through conceit, you will go on your way at peace. SN 8.4 ---------- ‘It is said, “the signless concentration of mind, the signless concentration of mind.” What now is the signless concentration of mind?’“Then, friends, it occurred to me: ‘Here, by nonattention to all signs, a bhikkhu enters and dwells in the signless concentration of mind. This is called the signless concentration of mind.’
“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 (nimittānusāri* viññāṇaṃ hoti).
*Anusari：[aor. of anusarati]: went after, followed - Anusarati [anu + sṛ] to follow - SK: √सृ sṛ : to run after, pursue (RV.)
Note: it is significant to realize that it is also at the level of sense-consciousness that the nimitta must be adressed. One must guard the doors of his sense faculties, AND also prevent consciousness to follow the nimitta, in the case that the former has not been properly performed.
I don’t know how reliable is MN 138, but one should take a look at this passage
Kathañcāvuso, bahiddhā viññāṇaṃ vikkhittaṃ visaṭanti vuccati?
Idhāvuso, bhikkhuno cakkhunā rūpaṃ disvā rūpanimittānusāri viññāṇaṃ hoti rūpanimittassādagadhitaṃ rūpanimittassādavinibandhaṃ rūpanimittassādasaṃyojanasaṃyuttaṃ bahiddhā viññāṇaṃ vikkhittaṃ (confused) visaṭanti (diffused, dispersed) vuccati.
“Then, friends, the Blessed One came to me by means of spiritual power and said this: ‘Moggallāna, Moggallāna, 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.
SN 40.9
"Monks, there are these three types of unskillful thinking: thinking of sensuality, thinking of ill will, thinking of harm. These three types of unskillful thinking cease without remainder in one who dwells with his mind well established in the four frames of reference or who develops the signless concentration. This is reason enough, monks, to develop the signless concentration. The signless concentration, when developed & pursued, is of great fruit, great benefit.
SN 22.80
And what, venerable sir, is the signless liberation of mind? Here, with nonattention to all signs, a bhikkhu enters and dwells in the signless concentration of mind. This is called the signless liberation of mind.
SN 41.7
Note: As one of the path to nibbana:
“And what, bhikkhus, is the path leading to the unconditioned?
Signless concentration….
SN 43.12
Ten children I bore from this physical heap. Then weak from that, aged, I went to a nun. She taught me the Dhamma: aggregates, sense spheres, & elements. Hearing her Dhamma, I cut off my hair & ordained.
Having purified the divine eye while still a probationer, I know my previous lives, where I lived in the past. I develop the sign-less meditation, well-focused oneness. I gain the liberation of immediacy — from lack of clinging, unbound. The five aggregates, comprehended, stand like a tree with its root cut through. I spit on old age. There is now no further becoming.
Thig 5.8
The one whose defilements are dried up,
Who’s not attached to food,
Whose resort is the liberation
That is signless and empty:
Their track is hard to trace,
Like that of birds in the sky.
Thag 1.92
Meditate on the signless,
Throw out the underlying tendency to conceit,
And when you have a breakthrough in understanding conceit,
You will live at peace.”
Thag 21.1
Metta.