Shatapatha Brahmana · Chapter 14.8.4

hṛdayabrahmopāsanaṃ brāhmaṇam
eṣa prajāpatiryaddhṛdayam/ etadbrahmaitatsarvaṃ tadetattryakṣaraṃ hṛdayamiti hṛ ityekamakṣaramabhiharantyasmai svāścānye ca ya evaṃ veda da ityekamakṣaraṃ dadantyasmai svāścānye ca ya evaṃ veda yamityekamakṣarameti svargaṃ lokaṃ ya evaṃ veda - १४.८.४