SB 6.19.11

yuvāṁ tu viśvasya vibhū
 jagataḥ kāraṇaṁ param
iyaṁ hi prakṛtiḥ sūkṣmā
 māyā-śaktir duratyayā
Word for word: 
yuvām — both of you; tu — indeed; viśvasya — of the universe; vibhū — the proprietors; jagataḥ — of the universe; kāraṇam — the cause; param — supreme; iyam — this; hi — certainly; prakṛtiḥ — energy; sūkṣmā — difficult to understand; māyā-śaktiḥ — the internal energy; duratyayā — difficult to overcome.
Translation: 
My Lord Viṣṇu and mother Lakṣmī, goddess of fortune, you are the proprietors of the entire creation. Indeed, you are the cause of the creation. Mother Lakṣmī is extremely difficult to understand because she is so powerful that the jurisdiction of her power is difficult to overcome. Mother Lakṣmī is represented in the material world as the external energy, but actually she is always the internal energy of the Lord.