The worship of the Shiva-Linga originated from the famous hymn in the Atharva-Veda Samhitâ sung in praise of the Yupa-Stambha, the sacrificial post.
In that hymn, a description is found of the beginingless and endless Stambha or Skambha, and it is shown that the said Skambha is put in place of the eternal Brahman.
