Skandapurana 16.107.33

16.107.33

syakāramaṃguṣṭhayornyasya dhīkāramurasi nyaset ..
makāraṃ jānumūle tu hi kāraṃ guhyamāśritam .. 33 ..

having placed the syllable 'sya' on the two big toes, he should place the syllable 'dhī' on the chest, / the syllable 'ma' at the root of the knees, and the syllable 'hi' resting on the private parts;