Shiva Purana
हंसस्य प्रतिलोमः स्यात्सकारार्थश्शिवः स्मृतः ॥ शक्त्यात्मको महामन्त्रवाच्यः स्यादिति निर्णयः ॥ ४१ ॥
It is the inverted form of Haṃsa. The meaning of the letter ‘Sa’ is Śiva. The conclusion is, the Śakti form is expressed by the great Mantra.
english translation
haMsasya pratilomaH syAtsakArArthazzivaH smRtaH ॥ zaktyAtmako mahAmantravAcyaH syAditi nirNayaH ॥ 41 ॥
hk transliteration by Sanscript