Shiva Purana
शंभुस्तन्मतमाज्ञाय तच्छिरांस्यच्युतेन ह ॥ पुरा छिन्नानि वै केतुसंज्ञानि निदधे गले ॥ १५॥
Realising his desire, Śiva fixed on to his neck his head that had been formerly cut off by Viṣṇu, and that was known as Ketu.
english translation
zaMbhustanmatamAjJAya tacchirAMsyacyutena ha ॥ purA chinnAni vai ketusaMjJAni nidadhe gale ॥ 15॥
hk transliteration by Sanscript