Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 1.3.28

1.3.28

athātaḥ pavamānānāmevābhyārohaḥ sa vai khalu prastotā sāma prastauti sa yatra prastuyāttadetāni japet | asato mā sadgamaya tamaso mā jyotirgamaya mṛtyormāmṛtaṃ gamayeti sa yadāhāsato mā sadgamayeti mṛtyurvā asatsadamṛtaṃ mṛtyormāmṛtaṃ gamayāmṛtaṃ mā kurvityevaitadāha tamaso mā jyotirgamayeti mṛtyurvai tamo jyotiramṛtaṃ mṛtyormāmṛtaṃ gamayāmṛtaṃ mā kurvityevaitadāha mṛtyormāmṛtaṃ gamayeti nātra tirohitamivāsti | atha yānītarāṇi stotrāṇi teṣvātmane'nnādyamāgāyettasmādu teṣu varaṃ vṛṇīta yaṃ kāmaṃ kāmayeta tam̐ sa eṣaevaṃ vidudgātātmane vā yajamānāya vā yaṃ kāmaṃ kāmayate tamāgāyati taddhaitallokajideva na haivālokyatāyā āśāsti ya evametatsāma veda

Next come the other Stotras with which the priest may obtain food for himself by singing them. Therefore let the sacrificer, while these Stotras are being sung, ask for a boon, whatever desire he may desire. An Udgâtri priest who knows this obtains by his singing whatever desire he may desire either for himself or for the sacrificer. This (knowledge) indeed is called the conqueror of the worlds. He who thus knows this Sâman, for him there is no fear of his not being admitted to the worlds.

He who knows what is the support of that Sâman, he is supported. Now verily its support is speech only. For, as supported in speech, that breath is sung as that Sâman. Some say the support is in food.
Next follows the Abhyâroha (the ascension) of the Pavamâna verses. Verily the Prastotri begins to sing the Sâman, and when he begins, then let him (the sacrificer) recite these (three Yagus-verses):
'Lead me from the unreal to the real! Lead me from darkness to light! Lead me from death to immortality!'
Now when he says, 'Lead me from the unreal to the real,' the unreal is verily death, the real immortality. He therefore says, 'Lead me from death to immortality, make me immortal.'
When he says, 'Lead me from darkness to light,' darkness is verily death, light immortality. He therefore says, 'Lead me from death to immortality, make me immortal.'
When he says, 'Lead me from death to immortality,' there is nothing there, as it were, hidden (obscure, requiring explanation).

1.3.28.p1

iti prathamāghyāye tṛtīyaṃ brāhmaṇam

He who knows what is the support of that Sâman, he is supported. Now verily its support is speech only. For, as supported in speech, that breath is sung as that Sâman. Some say the support is in food.
Next follows the Abhyâroha (the ascension) of the Pavamâna verses. Verily the Prastotri begins to sing the Sâman, and when he begins, then let him (the sacrificer) recite these (three Yagus-verses):
'Lead me from the unreal to the real! Lead me from darkness to light! Lead me from death to immortality!'
Now when he says, 'Lead me from the unreal to the real,' the unreal is verily death, the real immortality. He therefore says, 'Lead me from death to immortality, make me immortal.'
When he says, 'Lead me from darkness to light,' darkness is verily death, light immortality. He therefore says, 'Lead me from death to immortality, make me immortal.'
When he says, 'Lead me from death to immortality,' there is nothing there, as it were, hidden (obscure, requiring explanation).