CC Madhya 6.48

‘namo nārāyaṇāya’ bali’ namaskāra kaila
‘kṛṣṇe matir astu’ bali’ gosāñi kahila
Synonyms: 
namaḥ nārāyaṇāya — I offer my respects to Nārāyaṇa; bali’ — saying; namaskāra kaila — offered respects to Lord Caitanya Mahāprabhu; kṛṣṇe — unto Lord Kṛṣṇa; matiḥ astu — let there be attraction; bali’ — saying; gosāñi — Śrī Caitanya Mahāprabhu; kahila — spoke.
Translation: 
Offering his obeisances to Caitanya Mahāprabhu, Sārvabhauma Bhaṭṭācārya said, “Namo nārāyaṇāya” [“I offer my obeisances to Nārāyaṇa”].
Purport: 

In return, Caitanya Mahāprabhu said, “Kṛṣṇe matir astu” [“Let your attention be on Kṛṣṇa”].

It is the etiquette among sannyāsīs, those on the fourth platform of spiritual life, to offer respects by saying oṁ namo nārāyaṇāya (“I offer my respectful obeisances unto Nārāyaṇa”). This greeting is used especially by Māyāvādī sannyāsīs. According to the smṛti scriptures, a sannyāsī should not expect anything from anyone, nor should he consider himself identical with the Supreme Personality of Godhead. Vaiṣṇava sannyāsīs never think of themselves as being one with the Lord; they always consider themselves eternal servants of Kṛṣṇa, and they want to see everyone in the world become Kṛṣṇa conscious. For this reason, a Vaiṣṇava sannyāsī always offers his blessings to everyone, saying kṛṣṇe matir astu (“May you become Kṛṣṇa conscious”).