Ādityahṛdayamantra (Adityahridayamantra) is a powerful hymn in salutation to Āditya (Aditya), the Sun-god. The hymn consists of the salutation to Ghṛṇi(Ghrini) at first, then Sūrya (Surya), and then Āditya along with Praṇava(Pranava; Om-kara).
It is believed to be a sanātana (sanatana; eternal and traditional) hymn, which is also guhya( secret) as Sage Agastya says to Rāmacandra(Ramachandra). In Yuddhakāṇḍa(Yuddhakanda) of Rāmāyaṇa (Ramayana), when Rāma drove Rāvaṇa(Ravana) away from the battlefield, the latter, regaining consciousness, came back again. Seeing Rāvaṇa ready to fight again, Rāma, already tired after a long and exhausting battle, felt somewhat anxious. At that moment, Sage Agastya came to him and said,
Rāma Rāma mahāvāho sṛṇu guhyaṁ sanātanaṁ
yena sarvanārīn vatsa samare vijayiṣyase.
Ādityahṛdayaṁ puṇyaṁ sarvaśatruvināśanam
jayāvahaṁ japaṁ nityamakṣayaṁ paramaṁ śivam.
“O mighty-armed Rama, blissful for every heart, listen to the following eternal secret, in the form of a holy, eternal, immortal and supremely blessed and excellent encomium, entitled the Ādityahṛdayamantra, the bliss of all blessings, by means of which, my son, you will triumph over all your enemies on the battlefield, and which is believed to provide victory, root out all sins, allay all anxiety and grief once for all and prolonged life.” Rāma then chanted this hymn three times, facing the Sun, and got rid of his anxieties. After this he went to fight with renewed vigour and finally became victorious, slaying Rāvaṇa.