Sardhatrishatikalottara 6.2

6.2

śaktinyāsaṃ tato darbhair hṛdayenaiva kārayet
hṛdā vai śaktigarbhe tu prakṣipejjātavedasam //

then one should perform the placement of Śakti with kuśa grass by means of the hṛdayamantra. / With the hṛdayamantra, indeed, one should cast the fire into the womb of Śakti;