What's the origin of the verse "Mangalyam tantunanena mama jeevana hetuna...?"

Question

In many South Indian marriages while the groom is tying the maṅgalasūtra to the bride, the priest usually chants the following verse.

Mangalyam tantunanena mama jeevana hetuna:
kanthe badhnami subhage twam jeeva sarada satam

It's meaning from this blog:

This is a sacred thread. This is essential for my long life. I tie this around your neck, O maiden having many auspicious attributes may you live happily for a hundred years.

Word-for-word meaning:

Mangalyam thanthuna anena – This mangal sutra (This sacred thread) (thantu – thread)
hetuna – is essential (hetu – reason, purpose)
mama – my
jeevana – life (long life) (for my long life)
Kante – around your neck (in, on etc. prepositions are signified by the 'e' ending)
badhnami – I am tying
Subhage – oh, maiden of many auspicious attributes
twam – you (may you)
jeeva – live
sarada – years (from Sarad ritu)
satam – hundred

What's the origin of the verse? Which scripture contains this śloka?

Answer 1

In History of Dharmaśāstra, Vol. II Part I, P. V. Kane says that Śaunaka smṛti (a dharma śāstra work attributed to Śaunaka) is one of the earliest references to this mantra.

Maṅgalasūtra-bandhana: (tying a string having golden and other beads on it round the neck of the bride).

This is now regarded as the most important ornament which no woman will part with as long as her husband is alive. But the sūtras are entirely silent about it. Among the earliest references is one from Śaunaka smṛti¹²⁴² (ms. in Bombay University Library, folio 39 b). The Laghu-Āśvalāyana smṛti 15.33 also prescribes it and the mantra to be employed when doing so. Gadādhara on Pār. gṛ. I.8 says that maṅgala-sūtra should be worn and garlands be placed round their necks by the bride and bride-groom, though the sūtra of Pāraskara is silent on the point.

The Baud. gṛ. śeṣa sūtra V.5 in describing 'arkavivāha' speaks of māṅgalya sūtra to be tied to the plant. It is not clear whether it means the same as the maṅgalasūtra now tied by married women round their necks.

The footnote gives more details:

1242. ततो मङ्गलसूत्रं च ध्यात्वाभीष्टां च देवताम् । बद्धा कण्ठप्रदेशे स्याद्भूषणानि च शक्तितः ॥ शौनक;

      बन्धीयात्कन्यकाकण्ठे सूत्रं मणिसमन्वितम् ।
      माङ्गल्यतन्तुनानेन मन्त्रेण स्यात् सदा सती ॥ लघ्वाश्व 15.33.

      The मन्त्र is:
      माङ्गल्यतन्तुनानेन भर्तृजीवनहेतुना ।
      कण्ठे बध्नामि सुभगे सा जीव शरदः शतम् ॥.
      

      'सूत्रेऽनुपलब्धमपि वधूवरयोर्मङ्गलसूत्रं गले मालाधारणं' etc. गदाधर on पारस्कर I.1.

Answer 2

This mantra comes from the texts of Vaikhanasa Aagama, the first texts which gavethe ritual of marriage. They had mantras for performance of marriage where two people get married to another.