Ramayana
नाहत्वा समरे शत्रुं विष्णुः प्रतिनिवर्तते । दुर्लभश्चैव कामो ऽद्य वरगुप्ताद्धि रावणात् ॥ 7-27-18 ॥
Vishnu does not turn back from battle without having slain his enemy, and this desire of yours is hard to fulfill today against Ravana, who is protected by a boon.
english translation