Bhagavad Gita · 2.32

यदृच्छया चोपपन्नं स्वर्गद्वारमपावृतम्।सुखिनः क्षत्रियाः पार्थ लभन्ते युद्धमीदृशम्॥

yadṛichchhayā chopapannaṁ svarga-dvāram apāvṛitam sukhinaḥ kṣhatriyāḥ pārtha labhante yuddham īdṛiśham

O Partha, happy are the warriors who are called upon to fight in such a battle that comes unsought, opening the gates of heaven.

— Bhagavad Gita 2.32

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)