Bhagavatapurana 8.11.32

8.11.32

na tasya hi tvacamapi vajra ūrjito bibheda yaḥ surapatinaujaseritaḥ /
tadadbhutaṃ paramativīryavṛtrabhit tiraskṛto namuciśirodharatvacā //

32. But that strong vajra aimed indeed with great force by Indra did not injure even his (Namuchi's) skin; it was indeed most astonishing that the slayer of the very powerful *Vritra* was rendered ineffectual by the skin on the neck of Namuchi.