What is the verse when Sita warns Ravan of touching her in Lanka
This is what Sita says to Ravana -
तथा अहम् धर्म नित्यस्य धर्म पत्नी दृढ व्रता | त्वया संप्रष्टुम् न शक्या अहम् राक्षसाधम पापिना || ३-५६-१९
- raakshasa adhama = oh, demon, basely; tathaa = likewise - on par with Rama; dharma nityasya = in righteousness, one who steadied himself; dharma patnii = legitimate, wife; dR^iDha vrataa = solemnly, pledged [wife]; aham = such as I am; paapinaa tvayaa sampraSTum = by a trespasser, by you, to touch; aham na shakyaa = I am, not, possible - impossible.
"Likewise, I am the solemnly pledged legitimate wife of one who steadied himself in righteousness, and such as I am, you basely demon, it is impossible for you trespasser to touch me even. [3-56-19]
PS: I could not find this verse in the critical edition, though.
Sita was abducted by Ravana, on lifting her up with his left hand at her plait of hair at nape, and with his right hand at her thighs. So he did touch her during abduction.
इति उक्त्वा मैथिलीम् वाक्यम् प्रिय अर्हाम् प्रिय वादिनीम् || ३-४९-१५ अभिगम्य सुदुष्ट आत्मा राक्षसः काम मोहितः | जग्राह रावणः सीताम् बुधः खे रोहिणीम् इव || ३-४९-१६
On saying that sentence to Maithili, who by herself is a nice talker and who is a proper one for addressing with nice words, that verily evil-minded demon Ravana maddened by lust drew nigh of Seetha and grabbed her, as Budha, the Jupiter grabs the Star Rohini in firmament.
वामेन सीताम् पद्माक्षीम् मूर्धजेषु करेण सः | ऊर्वोः तु दक्षिणेन एव परिजग्राह पाणिना || ३-४९-१७
He that Ravana grabbed the lotus-eyed Seetha on lifting her up with his left hand at her plait of hair at nape, and with his right hand at her thighs.
However, Sita did warn him in Lanka (Sundara Kanda) that she can reduce him to ashes with austerity power.
असंदेशात्तु रामस्य तपसश्चामपालनात् | न त्वां कुर्मि दशग्रीव भस्म भर्मार्ह तेजसा || ५-२२-२०
"O Ravana! Although you are suited to be burnt into ashes, not having the mandate of Rama and preserving austerity, I am not reducing you into ashes with my glory."