Canto 2 -
Ayodhyā-kāṇḍa
Chapter 64: Daśaratha Gives up his Life in Remembrance of Lord Rāma
Text 2.64.55

त्वया तु यदविज्ञानान्निहतो मे सुतः शुचिः।
तेन त्वामभिशप्स्यामि सुदुःखमतिदारुणम्।।

tvayā tu yad avijñānān nihato me sutaḥ śuciḥ
tena tvām abhiśapsyāmi suduḥkham atidāruṇam

tvayā tu = you have; yat = because; avijñānāt = unknowingly; nihataḥ = killed; me = my; sutaḥ = son; śuciḥ = pure; tena tvām = you; abhiśapsyāmi = I will curse; suduḥkham = distressed; atidāruṇam = to become very terribly.

Because you have killed my pure son unknowingly, I will curse you to become very terribly distressed.1

When the sage said that he would curse him, he intended to say that he wouldn’t burn him up [immediately].

1 Technical note: tenaiva hetunā.