Valmiki Ramayana Ayodhya Kanda Sarga 20 Shloka 51
Original Shlokaस्थिरं हि नूनं हृदयं ममायसं न भिद्यते यद्भुवि नावदीर्यते। अनेन दुःखेन च देहमर्पितं ध्रुवं ह्यकाले मरणं न विद्यते।।2.20.51।।
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नूनम् assuredly, मम my, स्थिरम् still (hard), हृदयम् heart, आयसम् is made of iron, यत् since, न भिद्यते is not breaking, भुवि on this earth, नावदीर्यते does not disintegrate, अनेन this, दुःखेन due to sorrow, देहम् body, अर्पितम् surrendered (pervaded), अकाले untimely, मरणम् death, न विद्यते may not be possible, ध्रुवम् this is certain.
Surely my still (hard) heart is made of iron. It neither bursts nor breaks down on the ground. Pervaded by grief, there is no untimely death for this my body, too.