Congregational Takbeer in the mosque when hearing good news

When the Prophet
(peace and blessings of Allaah be upon
him) heard something that pleased him, he would say: Allaahu akbar, Allaahumma la
aysha ila aysh al-aakhirah (Allaah is most great, O Allaah, there is no
real life except the life of the Hereafter).

(Narrated by Ahmad). So it is permissible for a man to say Allaahu akbar when he sees
something that makes him glad, and he can raise his voice, without going to extremes. But
congergational Takbeer, where everyone starts and ends at the same time, is not prescribed
in Islam, because there is no report to indicate that. But if one person says Takbeer,
then another and another joins in, and they happen to say it at the same time, without
prior agreement, then there is nothing wrong with this. They can also prostrate (sajdat
al-shukr the prostration of thanksgiving) if it is a serious matter. And Allaah is
the Source of help.