CC Madhya 19.175
sādhana karile prema utpanna nā haya
bhukti — material enjoyment; mukti — to become liberated from material bondage; ādi — and so on; vāñchā — desires; yadi — if; mane — in the mind; haya — are; sādhana karile — even executing devotional service according to the regulative routine; prema — real love of Kṛṣṇa; utpanna — awakened; nā — not; haya — is.
“If one is infected with the desire for material enjoyment or material liberation, he cannot rise to the platform of pure loving service unto the Lord, even though he may superficially render devotional service according to the routine regulative principles.
Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura mentions that if one maintains within his heart the desire to enjoy the result of good work, or, being embarrassed by the material world, the desire to get out of material entanglement, one will never be able to attain the transcendental mellows of devotional service. In other words, one must not desire material profit when rendering devotional service. Even if one follows all the sixty-four regulative principles, he cannot attain pure devotional service with a contaminated heart.