Chandogya Upanishad 8.5.2

8.5.2

atha yat sattrāyaṇam ity ācakṣate brahmacaryam eva tat \
brahmacaryeṇa hy eva sata ātmanas trāṇaṃ vindate \
atha yan maunam ity ācakṣate brahmacaryam eva tat \
brahmacaryeṇa hy evātmānam anuvidya manute \\805.2\

2. Now, what people call ‘the protracted sacrifice’ (_sattrāyaṇa_) is really the chaste life of a student of sacred knowledge, for only through the chaste life of a student of sacred knowledge does one find the protection (_trāṇa_) of the real (_sat_) Soul (Ātman).

Now, what people call ‘silent asceticism’ (_mauna_) is really the chaste life of a student of sacred knowledge, for only in finding the Soul through the chaste life of a student of sacred knowledge does one [really] think (_manute_).