The phrase "to use the source" means to refer to or consult the original or main reference of information or data when doing research or gathering facts. Full definition
His monochromatic works explore the traditional portrait format on a range of scales, using source material from vintage postcards, magazines, found photos, and classic film stills. (artspace.com)
The vast majority of small businesses used those sources as seed money. (gapingvoid.com)
If I give you some source code, then any product created by using that source code is derived from my source code. (law.stackexchange.com)