PB5-926 is the sandy desert planet on which Apophis crashed after his escape from Sokar. SG-1 had received the gate coordinates as a radio transmission, and had traveled to the planet believing that the message had come from the Tok'ra. Instead they found an injured Apophis and brought him back to Earth as a prisoner. In doing so, Earth became a target of Sokar, who directed a particle beam through the stargate to pursue his prisoner and threatened Earth's destruction if Apophis was not returned.
Episode Reference: Serpent's Song
Stargate Coordinates: 12-36-23-18-7-27