Shiva Purana
एकाग्रमानसो योगी विमृश्यात्तां च मातृकाम् ॥ पुटितां प्रणवेनाथ न्यसेद्बाह्ये च मातृकाम् ॥ ५९ ॥
With a concentrated mind the yogi should meditate on that mother. O lord of the oṁkāra wrapped in a bag one should place the matrix on the outside.
english translation
ekAgramAnaso yogI vimRzyAttAM ca mAtRkAm ॥ puTitAM praNavenAtha nyasedbAhye ca mAtRkAm ॥ 59 ॥
hk transliteration by Sanscript