After a very quick and certainly not that comprehensive search on the interwebs, I can't find any source that says categorically that Corbyn did in fact vote to leave.
There are opinion pieces as to others having theories about how he voted, but nothing categorically stating either way.
Yes he was a Eurosceptic at times, but his opinions regarding Brexit changed often:
FactCheck: Corbyn’s changing Brexit stance – Channel 4 News