Once the download is complete, use a checksum tool to compare the file's MD5 hash against the one provided on the download page.
[Old version][Guide][Samsung] How to flash Stock ROM via ODIN
These are popular web databases where you can search by model number (e.g., SM-G991B) and region (CSC code). Note that free downloads on these sites often have speed limits.
: Older Samsung models often used a single .tar.md5 file for flashing. Newer models usually require four separate files (BL, AP, CP, and CSC), but can sometimes still be flashed using a single "HOME" file to preserve user settings. Where to Download