Śvetadvīpa-varṇana and Śāstra-pravartana (Śānti Parva 322)
आत्मना विदहितं दुःखमात्मना विहितं सुखम् | गर्भशय्यामुपादाय भुज्यते पौर्वदेहिकम्
bhīṣma uvāca | ātmanā vidhitaṃ duḥkham ātmanā vihitaṃ sukham | garbhaśayyām upādāya bhujyate paurvadehikam ||
ভীষ্ম বললেন: দুঃখ নিজেরই দ্বারা বিধৃত, সুখও নিজেরই দ্বারা বিধৃত। জীব গর্ভশয্যা গ্রহণ করামাত্রই পূর্বদেহে সঞ্চিত কর্মফল—সুখ-দুঃখ—ভোগ করতে শুরু করে।
भीष्म उवाच
That both suffering and happiness arise from one’s own actions: the self is the maker of its experience, and the results of past deeds (even from a previous embodiment) inevitably mature and are undergone.
In the Shanti Parva’s instruction on dharma and right understanding, Bhishma teaches Yudhishthira about karma and rebirth: the jīva, upon entering the womb, begins to experience the accumulated fruits of prior-life actions as pleasure and pain.