What is the scriptural reasoning behind the once save always saved belief?

From what I understand, It is the belief that once you have been "saved" by putting faith in Jesus, you are guaranteed your reward regardless of what you do from that point in your life short of becoming a satanic cultist. So it refers to being saved from receiving no reward, I guess.

it is a question that came up in a conversation with a colleague who couldnt find scriptures to back it up. I wondered if anyone here could help me with sources and understanding.

The Free Grace or non-traditional Calvinist doctrine has been espoused by Charles Stanley, Norman Geisler, Zane C. Hodges, Bill Bright, and others. This view, like the traditional Calvinist view, emphasizes that people are saved purely by an act of divine grace that does not depend at all on the deeds of the individual, and for that reason, advocates insist that nothing the person can do can affect his or her salvation.