Śānti-parva Adhyāya 30: Nārada–Parvata Samaya-bhaṅga, Śāpa, and the Marriage of Sukumārī
त्वयाहं प्रथमं शप्तो वानरस्त्वं भविष्यसि
tvayāhaṃ prathamaṃ śapto vānaras tvaṃ bhaviṣyasi
त्वयाहं प्रथमं शप्तो वानरस्त्वं भविष्यसि ॥
श्रीकृष्ण उवाच
The verse highlights moral causality in speech: initiating a harmful curse is not ethically neutral, and the agent becomes bound to consequences that can return upon them. It cautions restraint, responsibility, and dharmic discipline in words and intentions.
Kṛṣṇa addresses someone who had earlier cursed him, stating that because the other person was the first to cast the curse, that person will undergo a transformative consequence—becoming a monkey—presented as a direct result of the prior act.