Tevt 2121
The lord instructs Šrila Runa Gosvāmi i
288
śamaḥ —equilibrium;  mat —to Me;  niṣṭhatā —the quality of attachment;  buddheḥ —of the mind;  iti —thus;  śrī-mukha —from the mouth of the Supreme Lord;  gāthā —a verse.
TRANSLATION
“When one is fully attached to Kṛṣṇa’s lotus feet, one attains the śamatā stage. The word śamatā is derived from the word śama; therefore śānta-rasa, the position of neutrality, means being fully attached to the lotus feet of Kṛṣṇa. This is the verdict from the mouth of the Supreme Personality of God-head Himself. This state is called self-realization.
PURPORT
The word  śama  is explained by the Supreme Personality of Godhead in the following verse.
TEXT 212
শমো মন্নিষ্ঠত। বুদ্ধেরিতি শ্রীভগবহ্নচ: । তনিষ্ঠাদুর্ঘট। বুদ্ধেরেতাং শাস্তুরতিং বিন। ॥ ২১২ ॥
śamo man-niṣṭhatā buddher iti śrī-bhagavad-vacaḥ tan-niṣṭhā durghaṭā buddher etāṁ śānta-ratiṁ vinā
SYNONYMS
śāmaḥ  —equality or neutrality;  mat-niṣṭhatā  —being fixed in My lotus feet;  bud-dheḥ  —of the intelligence;  iti  —thus;  śrī-bhagavat-vacaḥ  —words of the Supreme Personality of Godhead;  tat-niṣṭhā  —attachment or attraction for Him;  durghaṭā  —very difficult to achieve;  buddheḥ  —of intelligence;  etām  —thus;  śānta-ratim  —attachment on the platform of  śānta-rasa ;  vinā  —without.
TRANSLATION
“These are the words of the Supreme Personality of Godhead: ‘When one’s intelligence is fully attached to My lotus feet but one does not render practical service, one has attained the stage called śānta-rati, or śama. Without śānta-rati, attachment to Kṛṣṇa is very difficult to achieve.’
PURPORT
This verse is found in  Bhakti-rasāmṛta-sindhu  (3.1.47).