What is the authenticity of the hadith: "The Hour will not be established until there is 'al-Mawat' among you"?

There is no hadith with the exact wording mentioned in the question. However, there are narrations indicating a high death toll before the Hour. In Bukhari, from Awf ibn Malik, the Prophet, peace and blessings be upon him, mentioned among the signs of the Hour: "My death, then the conquest of Jerusalem, then a widespread plague that will afflict you, like the Qas (a disease that kills sheep quickly)." Mawtan refers to widespread death. The Sunnah also indicates an increase in killing before the Hour. In Sahihayn, the Prophet, peace and blessings be upon him, said: "The Hour will not be established until al-harj becomes widespread," and al-harj was interpreted as killing.