Bhagavad Gita Chapter 9 Verse 23

Gita 9.23

ये, अपि, अन्यदेवताः, भक्ताः, यजन्ते, श्रद्धया, अन्विताः,
ते, अपि, माम्, एव, कौन्तेय, यजन्ति, अविधिपूर्वकम्।।23।।

Gita 9.23

Ye, api, anyadevtaH, bhaktaH, yajante, shraddhya, anvitaH,
Te, api, mam’, ev, kauntey, yajanti, avidhipoorvakam’ ||23||

Translation: (Kauntey) O Arjun! (shraddhya) with faith (anvitaH) endowed with (api) even (ye) those (bhaktaH) devotees (anyadevtaH) other gods (yajante) worship (te) they (api) also (mam’) me (ev) only (yajanti) worship, but their that worship (avidhipoorvakam’) is contrary to the rule i.e. against the scriptures. (23)

Gita 9.23: O Arjun! Even the devotees who worship other gods with faith, they also worship me only, but that worship of theirs is contrary to the rule i.e. against the scriptures.

Important: - Its evidence is given in Gita Chapter 16 Verse 23-24 that he who abandoning the injunctions of scriptures follow arbitrary (contrary to rule) way of worship, he neither attains ultimate peace, nor is his any task accomplished and nor does he attain supreme salvation i.e. is useless.