Yōga Sūtra 3.18

संस्कारसाक्षात्करणात्पूर्वजातिज्ञानम्।

saṁskārasākṣātkaraṇātpūrvajātijñānam।

[संस्कार साक्षात् करणात् पूर्व जाति ज्ञानम्।
saṁskāra = previous impressions; sākṣāt = direct; karaṇāt = by perception; pūrva = previous; jāti = births; jñānam = knowledge;]

By direct perception of previous impressions (and its samyama), one gets the knowledge of his previous births.