Mudgalapurana 2.29.50

2.29.50

khaṃ. 2 a. 30 pāna 62) gatvā vanāntaraṃ putra atapaṃ tapa uttamam |
ekākṣareṇa mantreṇa dhyātvā japaparāyaṇaḥ ||

[Khaṇḍa 2, Chapter 30, Page 62] Having gone to another forest, my son, I performed supreme austerities, / intent on recitation, having meditated with the single-syllable mantra.