Praśna Upaniṣad ("Six Questions") 5.3

5.3

sa yadyekamātramabhidhyāyīta sa tenaiva saṃveditastūrṇameva jagatyāmabhisaṃpadyate
tamṛco manuṣyalokamupanayante sa tatra tapasā brahmacaryeṇa śraddhayā saṃpanno mahimānamanubhavati

3. If he meditates on one element [namely *a*], having been instructed by that alone he quickly comes into the earth [after death]. The Rig verses lead him to the world of men. There, united with austerity, chastity, and faith, he experiences greatness.