So on what authority can we say that dwijas today MUST do sandhyavandana?
The oldest extant scripture to mandate Sandhyāvandana is probably the Āśvalāyana Gṛhyasūtra attached to Ṛgveda:
Adhyāya III, Kaṇḍikā 7
If the sun sets while he is sleeping without being sick, he should spend the rest of the night keeping silence, without sitting down, and should worship the sun (when it rises) with the five (verses), 'The light, O sun, by which thou destroyest darkness' (Rig-veda X, 37, 4 seq.).
If (the sun) rises (while he is sleeping without being sick), being fatigued without having done any work, or having done work that is not becoming, he should keep silence, &c., as before, and perform his worship (to the sun) with the following four (verses, Rig-veda X, 37, 9 seq.).
Invested with the sacrificial cord, constantly fulfilling the prescribed duties regarding the use of water, he should perform the Sandhyā (or twilight devotion), observing silence.
In the evening he should, turning his face to the north-west, to the region between the chief (west) point and the intermediate (north-western) point (of the horizon), murmur the Sāvitrī, (beginning) when the sun is half set, until the stars appear.
In the same way in the morning—
Standing, with his face turned to the east, until the disk (of the sun) appears.
If a dove flies against his house or towards it, he should sacrifice with (the hymn), 'O gods, the dove' (Rig-veda X, 165), verse by verse, or should murmur (that hymn).
'We have thee, O Lord of the path' (Rig-veda VI, 53)—if he is going out for doing some business.
'Bring us together, Pūṣan, with a knowing one' (Rig-Veda VI, 54)—if he wishes to find something lost, or if he has strayed.
'Journey over the ways, Pūṣan' (Rig-veda I, 42)—if he is going out on a long or dangerous way.
A similar rule is also found in Baudhāyana Dharmasūtra (Kṛsna Yajurveda):
Praśna II, Adhyāya 4, Kaṇḍikā 7
- Now, therefore, we will declare the rule for (performing) the twilight devotions.
...
- It is declared in the Veda, 'A Brāhmaṇa who in this same manner daily worships in the twilight, both at morn and at eve and, being sanctified by the Brahman, becoming one with the Brahman, and resplendent through the Brahman, follows the rules of the Śāstra, gains the heaven of Brahman.'
Manusmṛti also states it as compulsory (if you want to remain a dvija):
Section XIX - Twilight Prayers
Everyday during the earlier twilight one should stand repeating the
Sāvitrī, till the sun becomes distinctly visible; and during the later
twilight he should sit till the stars ark clearly seen.—(2.101)
One who, during the morning-twilight, repeats (the Sāvitrī) standing,
removes the sin of the (preceding) night; while he who, during the
evening-Twilight, repeats it seated, destroys the sin committed during
the day.—(2.102).
But he who does not stand during the morning-twilight, and who does
not sit through the evening-twilight, should be excluded, like the
Śūdra, from all that is due to twice-born persons.—(2.103)
Commenting on Manu 2.103, Medhātithi says:
The present verse, describing the evil accruing from the
non-performance of the Twilight-Prayers, serves to emphasise the
compulsory character of these.
He who does not keep standing during the morning-twilight and who does
not keep seated during the evening-twilight, should be regarded as a
Śūdra.
‘From all that is due to twice-born persons’;—i.e., entertaining as a
guest, honouring, offering of gifts and so forth.—‘He should be
excluded,’—i.e., discarded.
For this reason, in order to avoid being treated as a Śūdra one should
observe the Twilight Prayers every day.
