Mandukya Karika, verse 3.12


द्वयोर्द्वयोर्मधुज्ञाने परं ब्रह्म प्रकाशितम् ।
पृथिव्यामुदरे चैव यथाकाशः प्रकाशितः ॥ १२ ॥

dvayordvayormadhujñāne paraṃ brahma prakāśitam |
pṛthivyāmudare caiva yathākāśaḥ prakāśitaḥ || 12 ||

12. The description by pairs, as that of the Ākāśa, which is in the earth as also in the stomach (though referred to separately), applies equally to the Supreme Brahman described in the Madhu Brāhmaṇa (a chapter in the Bṛhadāraṇyaka Upaniṣad), as being both in the corporeal (Adhyātma) and in the celestial (Adhidaiva) regions.

Shankara Bhashya (commentary)

Moreover, in the words1 “All this is the Supreme Ātman, the Brahman, the bright, the immortal Person who is both the celestial (superphysical—Adhidaiva) and the corporeal (Adhyātma), who is in this earth as well as the Knower incorporated in the body,”—Brahman alone is described in order to indicate the limit at which duality vanishes. Where does this occur? It is thus replied:—It occurs in the Madhu Brāhmaṇa chapter which is known as the chapter dealing with the Knowledge of Brahman. It is because therein is described the nectar (i.e., immortality) which is known as Madhu, i.e., honey, as it gives us the highest bliss. This Brahman is like the Ākāśa which is said to be the same or identical though separately indicated as existing in the earth and in the stomach.