Update from the BBC:
Boris Johnson says Jeremy Corbyn "has become the first leader of the opposition in the
democratic history of our country to refuse the invitation of an
Prior to the Fixed-term Parliaments Act, the PM could call an election whenever (s)he wanted, so there was no need to invite the opposition to vote for one. It's therefore not so surprising that the 'democratic history of our country' has never previously need the leader of the opposition refuse such an invitation, because I doubt there's ever really been one...
If the Fixed-term Parliaments Act continues, then such refusals may be more common than one might expect, given that the point of the Act was to prevent PMs from calling elections at opportune moments.