Bhagavad Gita 9.23
येऽप्यन्यदेवता भक्ता यजन्ते श्रद्धयान्विताः। तेऽपि मामेव कौन्तेय यजन्त्यविधिपूर्वकम्।।9.23।।
ye'pyanyadevatā bhaktā yajante śraddhayānvitāḥ| te'pi māmeva kaunteya yajantyavidhipūrvakam||9.23||
ye'pyanyadevata bhakta yajante shraddhayanvitah te'pi mameva kaunteya yajantyavidhipurvakam9.23
Meaning
Even devotees who faithfully worship other gods actually worship Krishna alone, but they do so not by the prescribed method (avidhi-pūrvakam).
हे कौन्तेय! श्रद्धा से युक्त जो भक्त अन्य देवताओं को पूजते हैं, वे भी मुझे ही अविधिपूर्वक पूजते हैं।
This is verse 23 of 34 in Chapter 9 (Raja Vidya Raja Guhya Yoga) of the Bhagavad Gita.