Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 61

Sanskrit:

लब्ध्वा तदन्तरं शक्रो निद्रापहृतचेतस: ।दिते: प्रविष्ट उदरं योगेशो योगमायया ॥ ६१ ॥

ITRANS:

labdhvā tad-antaraṁ śakronidrāpahṛta-cetasaḥditeḥ praviṣṭa udaraṁyogeśo yoga-māyayā

Translation:

Finding this fault, Indra, who has all the mystic powers [the yoga-siddhis such as aṇimā and laghimā], entered Diti’s womb while she was unconscious, being fast asleep.

Purport:

A perfectly successful yogī is expert in eight kinds of perfection. By one of them, called aṇimā-siddhi, he can become smaller than an atom, and in that state he can enter anywhere. With this yogic power, Indra entered the womb of Diti while she was pregnant.