Shiva Purana
रामो यद्बलमाश्रित्य शिवदत्तश्च वै पुरा ॥ त्रिःसप्तकृत्वो नक्षत्रं ददाह हृषितो मुनिः ॥ २७ ॥
It was given to Rāma by Śiva formerly. Strengthened by it the delighted sage burnt the Kṣatriyas twenty-one times.
english translation
rAmo yadbalamAzritya zivadattazca vai purA ॥ triHsaptakRtvo nakSatraM dadAha hRSito muniH ॥ 27 ॥
hk transliteration by Sanscript