Valmiki Ramayana Bala Kanda Sarga 55 Shloka 21

तत: there after, पार्थिव: king, आश्रमपदम् towards hermitage (of Vasishta), गत्वा having gone, अस्त्राणि weapons, मुमोच released, यै: by their energy, तत् सर्वम् all that, तपोवनम् forest of ascetics (for carrying austerities ), अस्त्रतेजसा by the power of those weapons, निर्दग्धम् was burnt.

Thereafter king Viswamitra having proceeded towards the hermitage (of Vasishta) and released the weapons by the energy of which the entire forest of the ascetics was burnt down.