BṚHADĀRAṆYAKA UPANISHAD · 5.14.6

स य इमांस् त्रींल् लोकान् पूर्णान् प्रतिगृह्णीयात् सो ऽस्या एतत् प्रथमं पदम् आप्नुयात् ।
अथ यावतीयं त्रयी विद्या यस् तावत् प्रतिगृह्णीयात् सो ऽस्या एतद् द्वितीयं पदम् आप्नुयात् ।
अथ यावद् इदं प्राणि यस् तावत् प्रतिगृह्णीयात् सो ऽस्या एतत् तृतीयं पदम् आप्नुयात् ।
अथास्या एतद् एव तुरीयं दर्शतं पदं परोरजा य एष तपति ।
नैव केन चनाप्यम् ।
कुत उ एतावत् प्रतिगृह्णीयात् ॥

sa ya imāṃs trīṃl lokān pūrṇān pratigṛhṇīyāt so ’syā etat prathamaṃ padam āpnuyāt ।
atha yāvatīyaṃ trayī vidyā yas tāvat pratigṛhṇīyāt so ’syā etad dvitīyaṃ padam āpnuyāt ।
atha yāvad idaṃ prāṇi yas tāvat pratigṛhṇīyāt so ’syā etat tṛtīyaṃ padam āpnuyāt ।
athāsyā etad eva turīyaṃ darśataṃ padaṃ parorajā ya eṣa tapati ।
naiva kena canāpyam ।
kuta u etāvat pratigṛhṇīyāt ॥

If a man (a teacher) were to receive as his fee these three worlds full of all things, he would obtain that first foot of the Gâyatrî. And if a man were to receive as his fee everything as far as this threefold knowledge extends, he would obtain that second foot of the Gâyatrî. And if a man were to receive as his fee everything whatsoever breathes, he would obtain that third foot of the Gâyatrî. But 'that fourth bright foot, shining high above the skies ' cannot be obtained by anybody--whence then could one receive such a fee?

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)