Srimad-Bhagavatam: Canto 5 - Chapter 1 - Verse 43

Sanskrit:

अण्डमध्यगत: सूर्यो द्यावाभूम्योर्यदन्तरम् ।सूर्याण्डगोलयोर्मध्ये कोट्य: स्यु: पञ्चविंशति: ॥ ४३ ॥

ITRANS:

aṇḍa-madhya-gataḥ sūryodyāv-ābhūmyor yad antaramsūryāṇḍa-golayor madhyekoṭyaḥ syuḥ pañca-viṁśatiḥ

Translation:

The sun is situated [vertically] in the middle of the universe, in the area between Bhūrloka and Bhuvarloka, which is called antarikṣa, outer space. The distance between the sun and the circumference of the universe is twenty-five koṭi yojanas [two billion miles].

Purport:

The word koṭi means ten million, and a yojana is eight miles. The diameter of the universe is fifty koṭi yojanas (four billion miles). Therefore, since the sun is in the middle of the universe, the distance between the sun and the edge of the universe is calculated to be twenty-five koṭi yojanas (two billion miles).