For combustion to take place, a combustible material, an oxidizing agent, and an ignition source...

Spontaneous combustion is the exception to the requirement of a combustible material, an oxidizing agent, and an ignition source for combustion to take place. Spontaneous combustion is a phenomenon in which a material can ignite without any external heat or flame, due to a chemical reaction within the material that generates enough heat to reach the ignition temperature. Spontaneous combustion can occur in certain materials that are prone to self-heating, such as oily rags, hay, coal, and compost. Spontaneous combustion is a type of smoldering combustion, which is a slow, low-temperature, flameless form of combustion.References:

Fire Protection Handbook, 20th Edition, Volume 1, Chapter 2, Section 2.2.1.11

NFPA 921: Guide for Fire and Explosion Investigations, 2021 Edition, Section 4.3.12