In the Sermon on the Mount Jesus addressed the ostentatious prayers that some said in public in an effort to show off. What were these prayers?

According to Jewish custom, when the Israelites were in exile in Babylon they felt the loss of their thrice-yearly pilgramages to the temple in Jerusalem. In response, their leaders created a group of prayers that could, in part, take the place of their sacrifice and worship in the temple. 

Upon their arrival back in Jerusalem, a group of church leaders referred to as the Men of the Great Assembly (Hebrew: Anshe Kneset Ha-gedola) formalized these prayers as a part of daily Jewish worship. (Source 1, Source 2)

These prayers were extremely lengthy and (in its modern-day form at least) can take up to 45 minutes to say the morning prayer version. (Source 1)