There are many verses to support this. Tat te 'nukampāṁ su-samīkṣamāṇo bhuñjāna evātma-kṛtam (SB 10.14.8). A devotee, when he is put into distress, he thinks that "It is God's kindness that He is giving me little pain, hurt, although I should have suffered more." This is devotee's view. He is not, I mean to, disturbed by any kinds of pains and pleasure. Santuṣṭa. Satataṁ yogī. Even in distressed condition he also thinks of Kṛṣṇa. Therefore he is yogi. Yoginām api sarveṣāṁ mad-gatenāntar-ātmanā (BG 6.47). A devotee always thinks of Kṛṣṇa. That is devotee. Yogi. Yatātmā. He does not neglect his duty, his devotional service.
And dṛḍha-niścayaḥ. Dṛḍha-niścayaḥ means he believes in the word of Kṛṣṇa. Kṛṣṇa says, sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja (BG 18.66). A devotee believes in this, that "I have nothing to do, simply to surrender to Kṛṣṇa. Then my all business is done." That is confirmed in the Caitanya-caritāmṛta, kṛṣṇe bhakti kaile sarva-karma kṛta haya: "Simply by serving Kṛṣṇa, all other duties are discharged." This is called dṛḍha-niścayaḥ. Kṛṣṇa says, sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja (BG 18.66), and the devotee takes it very seriously. That is called dṛḍha-niścaya. Dṛḍha means firmly convinced, "Yes, if I surrender to Kṛṣṇa, all other business is complete." That is confirmed in the Caitanya-caritāmṛta.