What news did Gandalf receive that made him decide to rejoin Bilbo and the dwarves in The Hobbit?

We discover that Gandalf has re-entered the narrative
after leaving the dwarves at Beorn's house when Bilbo decides to steal the Arkenstone
and take it as bargaining power to Bard and the Elves to prevent a war. When he goes to
the Elvenking and Bard, Gandalf reveals himself, and although he does not tell Bilbo
straight away why he returned, we discover later that he clearly had heard of the
amassing of the armies of the Goblins and the Wargs. Gandalf chooses to reveal this news
to prevent a confrontation between the Dwarves and the elves and
men:

"Halt!"
he called in a voice like thunder, and his staff blased forth with a flash like the
lightning. "Dread has come upon you all! Alas! it has come more swiftly than I guessed.
The Goblins are upon you! Bolg of the North is coming, O Dain! whose father you slew in
Moria. Behold! the bats are above his army like a sea of locusts. They ride upon wolves
and Wargs are in their
train!"

With this news
Gandalf is able to divert the animosity and unite the Elves, men and Dwarves in the
Battle of the Five Armies.