In Vol. II of his exhaustive 1882 treatise “Legal Medicine“,  Charles Meymott Tidy, M.B., FCS, Master of Surgery; Professor of Chemistry and of Forensic Medicine and Public Health at the London Hospital, Medical Officer of Health for Islington, Late Deputy Medical Officer of Health and Public Analyst for the City of London, Etc., Etc., noted that:

of the spontaneous combustion either of heaps of damp leaves or of manure, no instance is recorded.

It is not clear what happened in the intervening 127 years, but we now have what is apparently a clear case. From the Los Angeles Times:

The Ventura County Sheriff’s Department said the fire appears to have been started by a “manure spontaneous combustion from a local ranch.”