Congregational Takbeer in the mosque when hearing good news
Praise be to Allaah.
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.