Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 43
Sanskrit:
तस्यापि दर्शयामास स्ववीर्यं जलकल्मष: ।यच्चकार गले नीलं तच्च साधोर्विभूषणम् ॥ ४३ ॥
ITRANS:
tasyāpi darśayām āsasva-vīryaṁ jala-kalmaṣaḥyac cakāra gale nīlaṁtac ca sādhor vibhūṣaṇam
Translation:
As if in defamation, the poison born from the Ocean of Milk manifested its potency by marking Lord Śiva’s neck with a bluish line. That line, however, is now accepted as an ornament of the Lord.