Shiva Purana
स्वहस्तम्भसितालिप्तं विन्यस्य शिशुमस्तके ॥ दक्षश्रुतावुपदिशेद्धंसस्सोहमिति स्फुटम् ॥ ४४ ॥
With his hand smeared with the ashes he shall touch the head of the pupil and instruct in his right ear clearly “Haṃsaḥ Soham.”
english translation
svahastambhasitAliptaM vinyasya zizumastake ॥ dakSazrutAvupadizeddhaMsassohamiti sphuTam ॥ 44 ॥
hk transliteration by Sanscript