Wiktionary
stubborn as a mule
a. (context simile English) Extremely stubborn
Wikipedia
Stubborn as a Mule
Stubborn as a Mule is a 2010 documentary directed by Arcelious Daniels & Miller Bargeron, Jr., which makes the case for reparations for slavery for African Americans.
Usage examples of "stubborn as a mule".
With a 'water brother' he is completely docile and with anybody else he is stubborn as a mule.