The Yerushalmi (Berachos 2, 4) says that Tishah B’Av is the birthday of Mashiach ben David.
I looked in Yerushalmi and could not find it.
Do you know where this statement can be located?
It is in Yerushalmi Brachos 17b of the old printings with the Pnei Moshe (not far from the end of Halacha 4). The top of the page has a story about someone who was plowing with an ox and the ox gave a scream, and after that another scream. An Arab came walking by and told him that the reason the ox screamed the first time was because Bais Hamikdash was destroyed and the second scream was because Moshiach was born. The Gemora later on in the middle of that page says from Rav Bon, “Why do we need this Arab, it is a posuk. The Posuk says Vhalivanon B’adir yipol, that the Bais Hamikdash will fall, and right after that it says that a king will come out of the brach of Yishai, which means that moshiach will be born immediately after the churban habayis. See Pnei Moshe on this gemora.