Canto 2 -
Ayodhyā-kāṇḍa
Chapter 34: Rāma, Lakṣmaṇa and Sītā Meet Daśaratha
Text 2.34.32

न हि सत्यात्मनस्तात धर्माभिमनसस्तव।
विनिवर्तयितुं बुद्धिः शक्यते रघुनन्दन॥

na hi satyātmanas tāta dharmābhimanasas tava
vinivartayituṁ buddhiḥ śakyate raghu-nandana

na = not; hi = for; satya-ātmanaḥ = You are truthful by nature 2; tāta = my dear child; dharma-abhimanasaḥ = and Your mind is absorbed in [following] dharma; tava = Your; vinivartayitum = to  change; buddhiḥ = decision; śakyate = it is possible; raghu-nandana = dear to the Raghus.

My dear child dear to the Raghus, it is not possible to change Your decision, for You are truthful by nature and Your mind is absorbed in [following] dharma.

NOTE. Why is Lord Śrī Rāma absorbed in [following] dharma? He Himself describes the reason for His meticulous adherence to Vedic dharma in Bhagavad-gītā 3.22-23:

na me pārthāsti kartavyaṁ triṣu lokeṣu kiñcana
nānavāptam avāptavyaṁ varta eva ca karmaṇi

“O son of Pṛthā, there is no work prescribed for Me within all the three planetary systems. Nor am I in want of anything, nor have I a need to obtain anything—and yet I am engaged in prescribed duties.”

yadi hy ahaṁ na varteyaṁ jātu karmaṇy atandritaḥ
mama vartmānuvartante manuṣyāḥ pārtha sarvaśaḥ

“For if I ever failed to engage in carefully performing prescribed duties, O Pārtha, certainly all men would follow My path.”