CC Madhya 25.153

ataeva bhāgavata karaha vicāra
ihā haite pābe sūtra-śrutira artha-sāra
Synonyms: 
ataeva — therefore; bhāgavata — Śrīmad-Bhāgavatam; karaha vicāra — try to understand scrutinizingly; ihā haite — from this; pābe — you will get; sūtra-śrutira — of the Vedic philosophy, the Brahma-sūtra; artha-sāra — the actual meaning.
Translation: 
Śrī Caitanya Mahāprabhu advised Prakāśānanda Sarasvatī, “Study Śrīmad-Bhāgavatam very scrutinizingly. Then you will understand the actual meaning of the Brahma-sūtra.”
Purport: 

Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura states that without studying Śrīmad-Bhāgavatam one cannot understand the purport of the Brahma-sūtra (Vedānta-sūtra) or the Upaniṣads. If one tries to understand Vedānta philosophy and the Upaniṣads without studying Śrīmad-Bhāgavatam, one will be bewildered and, construing a different meaning, will gradually become an atheist or an impersonalist.