Shiva Purana
यकारो ऽस्त्रं नमस्स्वाहा वषठुंवौषडित्यपि ॥ फडित्यपि च वर्णानामन्ते ऽङ्गत्वं यदा तदा ॥ ५३ ॥
The syllable y is the weapon, and the syllable namas svāhā is also used. Whenever the characters end in phadi and become a part of the character.
english translation
yakAro 'straM namassvAhA vaSaThuMvauSaDityapi ॥ phaDityapi ca varNAnAmante 'GgatvaM yadA tadA ॥ 53 ॥
hk transliteration by Sanscript