Sometimes it is spelled strung together with no hyphens.  Sometimes it is spelled with double Ss and spaces.  Twice, however, it is spelled with hyphens, and if I’m trying to be scrupulous about the hyphens, then here we are – Gollum’s signature sound:

• 05.066 S-s-s-s-s,’ hissed Gollum.
• 05.070 S-s-s-s-s,’ said Gollum

Both of these sounds are in the 1937 edition of The Hobbit, for those of you who have been tracking the differences between the elder and younger editions with me.