Shiva Purana
अश्वत्थामावतारस्ते वर्णितश्शंकर प्रभोः ॥ सर्वसिद्धिकरश्चापि भक्ताभीष्टफलप्रदः ॥ ४३ ॥
The incarnation in the form of Aśvatthāman of lord Śiva has been explained to you. It is the cause of all Siddhis and it fulfils all desires of the devotees.
english translation
azvatthAmAvatAraste varNitazzaMkara prabhoH ॥ sarvasiddhikarazcApi bhaktAbhISTaphalapradaH ॥ 43 ॥
hk transliteration by Sanscript