The scripture is Matthew 10:28:
"Do not be afraid of those who kill the body (soma) but cannot kill the soul (psukhén); fear him rather who can destroy both body and soul in Gehenna".
The passage could easily be read in a Platonic way (that the "soul" is the part of the person that cannot be "killed" and is contained in the body), but I don't think this is what the author intends. In the NT, the word psukhé is used in a Semitic manner similar to that of npsh in the OT (and psukhé in the LXX), namely, referring to the life of a person and the person as a living being (the Society's claim that the word means "body + spirit" is an oversimplification and wrong in many cases). Matthew 10:28 is perfectly consistent with this common usage of the term because people who KILL others succeed in killing the body but they cannot kill the person's life as it can be restored by God in the resurrection. The reference to Gehenna shows that the author's resurrection-judgment eschatology is here in view. The common Jewish eschatology is that in the end times, God would resurrect all the dead and judge them for their deeds and reward the righteous and punish the dead. Gehenna was one of the names in Jewish literature of the abode of the resurrected dead when they undergo eternal punishment. The reference to "both body and soul" confirms that it is the resurrected person that is being discussed (since resurrection is usually about embodiment). The idea then is: "Don't fear people who can kill you because it is a temporary killing of the body, as God will restore your soul in the resurrection. Instead, fear God since he can destroy in Gehenna both your resurrected body and your very life". People can only have mastery of one's body, God has mastery over life itself.
Sometimes the word psukhé was used in Jewish literature to refer to an immaterial soul that survives death (cf. Josephus, and some of the pseudepigrapha, and in Christian writings like Diognetus), but in the NT it only has this usage in Revelation. Paul never uses psukhé in this sense, tho he does hold an antithesis between pneuma "spirit" and soma or sarx "flesh", and he clearly does have an implicit body-soul dualism in 2 Corinthians 5:1-10, 12:2 and Philippians 1:20-24 (cf. also 2 Peter 1:13-14), and even uses certain technical terms like "naked" and "tent" which also occur in Platonic writings to refer to the immortal soul.