सन्ध्याहीनः अशुचिः नित्यम् अनर्हहः सर्वकर्मसु।
यदन्यत् कुरुते कर्म न तस्य फलभाग् भवेत्॥
– दक्षस्मृति २-२७
sandhyāhīnaḥ aśuciḥ nityam anarhahaḥ sarvakarmasu|
yadanyat kurute karma na tasya pha1labhāg bhavet||
– dakṣasmṛti 2-27
One who does not perform Sandhya is always impure and unfit for performing any other rites. Whatever other rites he may perform, he will not obtain the fruit thereof.
I first came across this quote in an upanyāsam of Bhagavad Gitā by Shri Velukkudi Krishnan, in a discussion of the 16th Chapter, detailing Asura-svabhavā and Daiva-svabhāva. Sandhyāvandana is THE most important karma of every dvija, and must be performed even under the direst of circumstances. This verse also clearly states that there is no use in performing any other rites, for a person who does not perform his Sandhyā.