Matsya Purana — Characteristics of Dvāpara and Kali Yugas
संस्थिता सहसा या तु सेना प्रमतिना सह गङ्गायमुनयोर्मध्ये सिद्धिं प्राप्ता समाधिना //
saṃsthitā sahasā yā tu senā pramatinā saha gaṅgāyamunayormadhye siddhiṃ prāptā samādhinā //
But that army, suddenly coming to a halt together with Pramati, attained spiritual accomplishment (siddhi) through samādhi in the region between the Gaṅgā and the Yamunā.
This verse does not discuss Pralaya; it highlights the tirtha-power of the Ganga–Yamuna region, where samādhi leads to siddhi.
It implies that disciplined stillness and meditative practice—even for a group engaged in worldly action—can be redirected toward dharmic attainment when undertaken in a sanctified place.
No Vāstu/temple rule is stated; the ritual takeaway is the efficacy of entering samādhi at a recognized tirtha (between Gaṅgā and Yamunā) to gain siddhi.