Brahmanda Purana — Anushanga Pada, Shloka 1
Jāmadagnya-Rāmasya Tapaścaraṇam
The Austerities of Rama Jamadagnya
इति श्रीब्रह्माण्डे महापुराणे वायुप्रोक्ते मध्यभागे तृतीये उपोद्धातपादे वसिष्ठसगरसंवादे अर्चुनोपाख्याने जामदग्न्यतपश्चरणं नाम द्वाविंशतितमो ऽध्यायः // २२// वसिष्ठ उवाच तपस्विनं तदा राममेकाग्रमनसं भवे / रहस्येकान्तनिरतं नियतं शंसितव्रतम्
iti śrībrahmāṇḍe mahāpurāṇe vāyuprokte madhyabhāge tṛtīye upoddhātapāde vasiṣṭhasagarasaṃvāde arcunopākhyāne jāmadagnyatapaścaraṇaṃ nāma dvāviṃśatitamo 'dhyāyaḥ // 22// vasiṣṭha uvāca tapasvinaṃ tadā rāmamekāgramanasaṃ bhave / rahasyekāntanirataṃ niyataṃ śaṃsitavratam
Thus, in the Śrī Brahmāṇḍa Mahāpurāṇa, in the middle section spoken by Vāyu, in the third Upoddhāta-pāda, within the dialogue of Vasiṣṭha and Sagara, in the Arjuna episode, is the twenty-second chapter called “The Austerity-practice of Jāmadagnya.” Vasiṣṭha said: then the ascetic Rāma was single-pointed in mind, devoted to secret solitude, disciplined, and steadfast in praised vows.