Nimittasutta

“Adhicittamanuyuttena, bhikkhave, bhikkhunā tīṇi nimittāni kālena kālaṁ manasi kātabbāni—

kālena kālaṁ samādhinimittaṁ manasi kātabbaṁ, kālena kālaṁ paggahanimittaṁ manasi kātabbaṁ, kālena kālaṁ upekkhānimittaṁ manasi kātabbaṁ.

Sace, bhikkhave, adhicittamanuyutto bhikkhu ekantaṁ samādhinimittaṁyeva manasi kareyya, ṭhānaṁ taṁ cittaṁ kosajjāya saṁvatteyya.  
Foundations

“Mendicants, a mendicant committed to the higher mind should focus on three foundations from time to time:

the foundation of immersion, the foundation of exertion, and the foundation of equanimity.

If a mendicant dedicated to the higher mind focuses solely on the foundation of immersion, it’s likely their mind will incline to laziness.

“‘Animitto cetosamādhi, animitto cetosamādhī’ti vuccati.

Katamo nu kho animitto cetosamādhīti?

Tassa mayhaṁ, āvuso, etadahosi:

‘idha bhikkhu sabbanimittānaṁ amanasikārā animittaṁ cetosamādhiṁ upasampajja viharati.  
“They speak of this thing called the ‘signless immersion of the heart’.

What is the signless immersion of the heart?

It occurred to me:

‘It’s when a mendicant, not focusing on any signs, enters and remains in the signless immersion of the heart.