Accidentally rewrote a function from R base package, how to get back to default?

Accidentally rewrote a function from R base package, how to get back to
default?

I have accidentally rewrote which() function from base package in R by my
own code. I need the original default which() function really badly but
made a mistake of saving my workspace ( R Console) with the new (my own)
definition of which() function ( that ironically uses the original which()
function in it) so now which doesn't work at all.. How do I get the
default which() back? Is there a way to find the source code for which()
function?