Varaha Purana 182.21

तत एतेन मन्त्रेण प्रासादं संप्रवेशयेत् ॥ प्रतिमा स्थापितव्या मे मध्ये न तु विपार्श्वतः ॥

tata etena mantreṇa prāsādaṁ saṁpraveśayet || pratimā sthāpitavyā me madhye na tu vipārśvataḥ ||

Then, with this mantra, one should enter the temple. The image (pratimā) is to be installed for Me in the center, not to either side.