SB 10.1.59

dṛṣṭvā samatvaṁ tac chaureḥ
 satye caiva vyavasthitim
kaṁsas tuṣṭa-manā rājan
 prahasann idam abravīt
Word for word: 
dṛṣṭvā — by seeing; samatvam — being equipoised, undisturbed in distress or happiness; tat — that; śaureḥ — of Vasudeva; satye — in truthfulness; ca — indeed; eva — certainly; vyavasthitim — the firm situation; kaṁsaḥ — Kaṁsa; tuṣṭa-manāḥ — being very satisfied (with Vasudeva’s behavior in delivering the first child to keep his promise); rājan — O Mahārāja Parīkṣit; prahasan — with a smiling face; idam — this; abravīt — said.
Translation: 
My dear King Parīkṣit, when Kaṁsa saw that Vasudeva, being situated in truthfulness, was completely equipoised in giving him the child, he was very happy. Therefore, with a smiling face, he spoke as follows.
Purport: 

The word samatvam is very significant in this verse. Samatvam refers to one who is always equipoised, unaffected by either happiness or distress. Vasudeva was so steadily equipoised that he did not seem in the least agitated when delivering his first-born child into the hands of Kaṁsa to be killed. In Bhagavad-gītā (2.56) it is said, duḥkheṣv anudvigna-manāḥ sukheṣu vigata-spṛhaḥ. In the material world, one should not be very eager to be happy, nor should one be very much disturbed by material distress. Lord Kṛṣṇa advised Arjuna:

mātrā-sparśās tu kaunteya
 śītoṣṇa-sukha-duḥkha-dāḥ
āgamāpāyino ’nityās
 tāṁs titikṣasva bhārata

“O son of Kuntī, the nonpermanent appearance of happiness and distress, and their disappearance in due course, are like the appearance and disappearance of winter and summer seasons. They arise from sense perception, O scion of Bharata, and one must learn to tolerate them without being disturbed.” (Bg. 2.14) The self-realized soul is never disturbed by so-called distress or happiness, and this is especially true of an exalted devotee like Vasudeva, who showed this by his practical example. Vasudeva was not at all disturbed when delivering his first child to Kaṁsa to be killed.