Canto 2 -
Ayodhyā-kāṇḍa
Chapter 61: Kausalyā Condemns Daśaratha in Extreme Distress
Text 2.61.9

वज्रसारमयं नूनं हृदयं मे न संशयः।
अपश्यन्त्या न तं यद्वै फलतीदं सहस्रधा।।

vajra-sāramayaṁ nūnaṁ hṛdayaṁ me na saṁśayaḥ
apaśyantyā na taṁ yad vai phalatīdaṁ sahasradhā

vajra-sāramayam = as hard as the thunderbolt; nūnam = certainly; hṛdayam = heart is; me = my; na saṁśayaḥ = undoubtedly; apaśyantyāḥ = upon not seeing; na = not; tam = Him; yat vai = for; phalati = does burst; idam = it; sahasradhā = into a thousand pieces.

Undoubtedly my heart is certainly as hard as the thunderbolt for it does not burst into a thousand pieces upon not seeing Him.