I read several times the Ramayana (I can read books, specially long ones, untold number of times, because I forget everything on them instantly) but I cannot recall a scene of Hanuman fighting a bull -- although Ravana may have shape-shifted into a bull during a fight.
I found the attached image of this statue on the internet.It is in Kampong Cham Town, in Cambodia,reportedly illustrating "Hanuman fighting a bull".
An also perhaps a related story is that of Vali, the king of the monkeys, killing a demon called Dundubhi, which being 'horribly dark and hairy', can be pictured as a buffalo.. Vali, king of the monkeys and elder brother of Sugriva, was eventually killed by Sugriva's ally Rama, who sacred him in turn as king of the monkeys. Hanuman was Sugriva's top military advisor.