Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 31

Sanskrit:

स तं प्रविष्टं वृतमाततायिभि-र्भटैरनीकैरवलोक्य माधव: ।उद्यम्य मौर्वं परिघं व्यवस्थितोयथान्तको दण्डधरो जिघांसया ॥ ३१ ॥

ITRANS:

sa taṁ praviṣṭaṁ vṛtam ātatāyibhirbhaṭair anīkair avalokya mādhavaḥudyamya maurvaṁ parighaṁ vyavasthitoyathāntako daṇḍa-dharo jighāṁsayā

Translation:

Seeing Bāṇāsura enter with many armed guards, Aniruddha raised His iron club and stood resolute, ready to strike anyone who attacked Him. He resembled death personified holding his rod of punishment.

Purport:

The club was not made of ordinary iron but of a special kind called muru.