Valmiki Ramayana Ayodhya Kanda Sarga 40 Shloka 23

देवगर्भप्रतिमे resembling the offspring of gods, वनम् to the forest, याति while going, यत् for what reason, न भिद्यते does not break, नूनम् certainly, राममातुः of the mother of Rama (Kausalya's), हृदयम् heart, आयसम् is made of iron, असंशयम् no doubt.

Alas, the heart of Kausalya whose son, Rama, resembles the offspring of the gods, does not break even though he is going to the forest Undoubtedly it must be made of iron.