CC Madhya 24.140
kliśyanti ye kevala-bodha-labdhaye
teṣām asau kleśala eva śiṣyate
nānyad yathā sthūla-tuṣāvaghātinām
śreyaḥ-sṛtim — the auspicious path of liberation; bhaktim — devotional service; udasya — giving up; te — of You; vibho — O my Lord; kliśyanti — accept increased difficulties; ye — all those persons who; kevala — only; bodha-labdhaye — for obtaining knowledge; teṣām — for them; asau — that; kleśalaḥ — trouble; eva — only; śiṣyate — remains; na — not; anyat — anything else; yathā — as much as; sthūla — bulky; tuṣa — husks of rice; avaghātinām — of those beating.
“ ‘My dear Lord, devotional service unto You is the only auspicious path. If one gives it up simply for speculative knowledge or the understanding that these living beings are spirit souls and the material world is false, he undergoes a great deal of trouble. He only gains troublesome and inauspicious activities. His actions are like beating a husk that is already devoid of rice. His labor becomes fruitless.’
This is a quotation from Śrīmad-Bhāgavatam (10.14.4).