Sandhyavandana (Sanskrit: संध्यावन्दन, sandhyāvandana) is a mandatory religious ritual performed, traditionally, by Dvija communities of Hindus, particularly those initiated through the sacred thread ceremony referred to as the Upanayanam and instructed in its execution by a Guru, in this case one qualified to teach Vedic ritual.

The Sandhyāvandanam consists of recitation from the Vedas, accompanied by ritual. These rituals are performed three times a day - at morning (prātaḥsaṃdhyā), noon (mādhyāhnika) and evening (sāyaṃsaṃdhyā).

However, a question arises, as to how the practice of performing Sandhyavandana, three times a day, started.

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Rig Veda III.4.2, says as follows:

यं देवासस्त्रिरहन्नायजन्ते दिवे-दिवे वरुणो मित्रो अग्निः | सेमं यज्ञं मधुमन्तं कर्धी नस्तनूनपाद घर्तयोनिं विधन्तम ||

yaṃ devāsastrirahannāyajante dive-dive varuṇo mitro aghniḥ | semaṃ yajñaṃ madhumantaṃ kṛdhī nastanūnapād ghṛtayoniṃ vidhantam ||

Tanūnapāt whom the deities - MITRA, VARUNA and AGNI, worship daily thrice a day, render this our sacred rain-engendering sacrifice productive of water.

The translation is of H.H. Wilson.

The composer of the mantra was Sage Viswamithra.

So this Rig Vedic practice might be a precursor to the practice of performing Sandhyavandana, three times a day, in the subsequent period.

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A peculiar aspect is that Mandala 2 of the Rig Veda, which pre-dated the above mantra, mentions about worshiping AGNI, only twice a day.

Rig Veda II.8.3,composed by the sage from Bhrigu family, eulogises AGNI as follows:

य उ शरिया दमेष्वा दोषोषसि परशस्यते | यस्य वरतं न मीयते ||

Who for his glory is extolled at eve and morning in our homes, Whose statute is inviolate;

This indicates that by the time this mantra was composed, AGNI had been worshiped twice a day only.