SB 8.11.33

tasmād indro ’bibhec chatror
 vajraḥ pratihato yataḥ
kim idaṁ daiva-yogena
 bhūtaṁ loka-vimohanam
Word for word: 
tasmāt — therefore; indraḥ — the King of heaven; abibhet — became very fearful; śatroḥ — from the enemy (Namuci); vajraḥ — the thunderbolt; pratihataḥ — was unable to hit and returned; yataḥ — because; kim idam — what is this; daiva-yogena — by some superior force; bhūtam — it has happened; loka-vimohanam — so wonderful to the people in general.
Translation: 
When Indra saw the thunderbolt return from the enemy, he was very much afraid. He began to wonder whether this had happened because of some miraculous superior power.
Purport: 

Indra’s thunderbolt is invincible, and therefore when Indra saw that it had returned without doing any injury to Namuci, he was certainly very much afraid.