Skanda Purana — Nagara Khanda, Shloka 19
मन्त्रराजं सप्रणवं ध्यायेत्सोऽपि न पश्यति । इत्युक्ता सा तपोनिष्ठा तपश्चरितुमागता
mantrarājaṃ sapraṇavaṃ dhyāyetso'pi na paśyati | ityuktā sā taponiṣṭhā tapaścaritumāgatā
たとえプラナヴァ（オーム）を伴う「真言の王」を観想しても、それだけで究極を見得ることはない。かく教えられ、苦行に堅固なる彼女は、タパスを修するために赴いた。
Narrator within Brahma–Nārada dialogue (attribution by section context)
Scene: A teacher-figure instructs an ascetic woman; behind them, a symbolic praṇava and mantra appear, while the ascetic turns toward the path of austerity in the mountains.
FAQs
Mantra and meditation must be supported by disciplined tapas and steadfast conduct to bear fruit.
The setting is within the Hāṭakeśvara-kṣetra Māhātmya section of the Nāgara-khaṇḍa.
Meditation on the mantrarāja with Praṇava is referenced, alongside the necessity of performing tapas.