Shiva Purana
मात्राक्रमेण विज्ञेयाश्चोद्वातक्रमयोगतः ॥ नाडीविशुद्धिपूर्वं तु प्राणायामं समाचरेत् ॥ ३२ ॥
The duration of Prāṇāyāma shall be increased in accordance with the Mātrās and strokes. The veins shall be necessarily purified.
english translation
mAtrAkrameNa vijJeyAzcodvAtakramayogataH ॥ nADIvizuddhipUrvaM tu prANAyAmaM samAcaret ॥ 32 ॥
hk transliteration by Sanscript