Revākhaṇḍa ascribed to the Vāyupurāṇa (RKV) 11.17

11.17

narmadāyāḥ punastīre kṣipraṃ siddhiravāpyate /
ṣaḍbhirvarṣaistu sidhyanti ye tu sāṃkhyavido janāḥ //

but on the bank of the Narmadā, success is attained quickly. / Those people who know Sāṃkhya achieve success in six years;