Aitareya Upanishad 2.5

तदस्य द्वितीयं जन्म | तदस्यात्मानं पुण्येभ्यः कर्मभ्यः प्रतिदधाति | अथास्यायमात्मा कृतकृत्यो वयोगतः प्रैति | स इतः प्रयन्पुनरेव जायते तदस्य तृतीयं जन्म ॥

tad asya dvitiyam janma | tad asyatmanam punyebhyah karmabhyah pratidadhati | athasyayam atma krita-krityo vayo-gatah praiti | sa itah prayan punar eva jayate tad asya tritiyam janma ||

That is his second birth. This self of his is placed for the performance of virtuous deeds. Then this other self, having fulfilled his duties and reached old age, departs. Departing hence, he is born again. That is his third birth.

यह उसका द्वितीय जन्म है। वह (पिता) अपने इस आत्मा (पुत्र) को पुण्य-कर्मों के निमित्त नियुक्त करता है। तदनन्तर यह उसका (पिता का) आत्मा कृतकृत्य होकर, वयःप्राप्त होकर, (इस लोक से) प्रयाण करता है। यहाँ से प्रयाण करके वह पुनः जन्म लेता है — यही उसका तृतीय जन्म है।