Valmiki Ramayana Ayodhya Kanda Sarga 64 Shloka 54
Original Shlokaत्वया तु यदविज्ञानान्निहतो मे सुतश्शुचिः। तेन त्वामभिशप्स्यामि सुदुःखमतिदारुणम्।।2.64.54।।
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त्वया by you, अविज्ञानात् in ignorance, मे my, शुचिः purehearted, सुतः son, यत् since, निहतः was killed, तेन therefore, त्वाम् you, सुदुःखम् deeply sorrowful, अतिदारुणम् extremely dreadful, अभिशप्स्यामि I will curse.
'Though you have killed my purehearted son unintentionally, I will pronounce a disastrous and dreadful curse on you.