SB 10.62.31
sa tam pravistam vrtam atatayibhir
bhatair anikair avalokya madhavah
udyamya maurvam parigham vyavasthito
yathantako danda-dharo jighamsaya
Translation:
Seeing Banasura 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.