It is mentioned in Manusmriti in the 181th line that "A twice-born student, who has involuntarily wasted his manly strength during sleep, must bathe, worship the sun, and afterwards thrice mutter the Rik-verse (which begins), 'Again let my strength return to me.'" Which verse is described here? https://www.sacred-texts.com/hin/manu/manu02.htm
Explanatory notes by Ganganath Jha in his translation of Manusmṛti (with Medhātithi's Manubhāṣya) say that Manusmṛti 2.181 is referring to Taittirīya Āraṇyaka 1.30 (of Kṛṣṇa Yajurveda):
'Punarmām' – "This verse occurs in Taittirīya Āraṇyaka 1.30," – Buhler.
'Punarmāmaitvindriyam' – "Taittirīya Āraṇyaka 1.30. Such uses of texts are frequent in the later Vedic works; e.g. the Sāmavidhāna Brāhmaṇa and the several Ṛgvidhānas," – Burnell.
From here, the verse in full:
punarāyuḥ punarbhagaḥ |
punarbrḥhmaṇamaitu mā |
punardraviṇamaitu mā |
yanme'dya retaḥ pṛthivīmaskān |
idam tatpunarādade |
dīrghāyutvāya varcase |
yanme retaḥ prasicyate |
yanma ājāyate punaḥ |
tena māmamṛtaṃ kuru |
tena suprajasaṃ kuru ||1||
punardve ca ||
prathamaprapāṭhake triṃśo'nuvākaḥ ||30||
Translation and commentary of the verse(s) from Essence of Taittiriya Aranyaka (kamakoti.org):
1.30.1-3: May mantra shakti lead to physical fulfillment and good progeny
Punarmaamaittvindriyam punaraayuh punarbhagaha,
punarbraahmanamaitu maa punardravinamaitu maa /
Yanme~dya retah prithiveemaskaan yadoshadheerapyasaradyaapaha
idamtatpunaraadade deerghaayuttvaaya varchase /
Yanme retah prasichyate | yanma aajaayate punaha | tena maamamrutam kuru | tena suprajasankuru /
This anuvaka deals with human desires and their fulfillment. The prayers are for awakening senses, their strength and intensities. The prayers are to revive and reinforce the world of aspirations and thus the frontiers of life span, the power of enjoyment, their sustenance and enhancement.
May the mantras of prosperity and longevity to enjoy the sweet fruits of life respond instantly. May the "retas" or semen be spilled on earth and enhance the derivatives of plants, progeny and pashus. Besides these again, the long life, the capacity to enjoy and the determination towards fulfillment be heightened further and farther.
Finally, may the semen deposited in the womb of the wife result in many sons with name and fame besides our immortality!