Shiva Purana
प्राश्याथ प्रणवेनैव द्विराचम्याथ संस्पृशेत् ॥ नाभिं मन्त्रान्वक्ष्यमाणन्प्रणवाद्यान्नमोन्तकान् ॥ ९५ ॥
After drinking the oṁkāra one should perform the Dvirachama and then touch the body. Reciting the mantras to the navel and chanting the oṁkāras and offering obeisances.
english translation
prAzyAtha praNavenaiva dvirAcamyAtha saMspRzet ॥ nAbhiM mantrAnvakSyamANanpraNavAdyAnnamontakAn ॥ 95 ॥
hk transliteration by Sanscript