When the son of the brāhmaṇa [Aśvatthāmā] saw that his horses were tired, he considered that there was no alternative for protection outside of his using the ultimate weapon, the brahmāstra [nuclear weapon].
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
यदाशरणमात्मानमैक्षत श्रान्तवाजिनम् । अस्त्रं ब्रह्मशिरो मेने आत्मत्राणं द्विजात्मजः
yadāśaraṇamātmānamaikṣata śrāntavājinam | astraṃ brahmaśiro mene ātmatrāṇaṃ dvijātmajaḥ
SB 1.7.19