Skanda Purana 4.41.236

4.41.236

prāṇaṃ tatra vinīya paṃcaghaṭikaṃ ciṃtānvitaṃ dhārayedeṣā mokṣakapāṭapāṭanapaṭuḥ proktā nabhodhāraṇā .. 17 ..

having led the breath there, one should hold it, united with thought, for five ghaṭikās; this is declared as the retention of space, capable of splitting open the doors of liberation. (17)