Sans.

"sans." is a frequently ripped track from Undertale and .

Summary
"sans.", titled "mus_muscle" in the game files, is a track that plays in many locations throughout Undertale, mostly relating to the eponymous character: it plays during Sans' introduction in Snowdin Forest, in rooms containing his sentry stations, inside Grillby's, in Papyrus and Sans' house, and in Napstablook's snail farm. Its melody is also used as a leitmotif in "It's Raining Somewhere Else", a part of "Bring It In, Guys!", and the unused "Song That Might Play When You Fight Sans". The track was later reused in Deltarune, where it plays in Sans' convenience store in Hometown.

Toby Fox originally composed the song sometime in October 2012 for use in a project related to Homestuck, a webcomic he was a major contributor to. It was originally titled "muscles.mp3", explaining the track's filename in Undertale.

Rips
The first rip of "sans.", "sans. - Undertale", was uploaded to the GiIvaSunner channel on March 28, 2016. After the GiIvaSunner Termination, the rip was reuploaded to the SiIvaGunner channel on May 2, 2016. As of September 22, 2021, this rip has garnered over 805 thousand views, making it the most viewed rip of "sans." on the channel.

There are currently rips of this song on the channel, including "muscles (Unused) - Hiveswap", which references the song's original planned use in a Homestuck-related project.