What is the authenticity of the Hadith stating that the Messenger of Allah, peace and blessings be upon him, combined the Dhuhr and Asr prayers, and the Maghrib and Isha prayers, without fear, rain, or travel? What is its meaning, and do the Shiites use it as an argument for shortening prayers when not traveling?

The aforementioned Hadith is authentic, narrated by Al-Bukhari, Muslim, and others from Ibn 'Abbas. It is not permissible to use it as evidence for shortening prayers (qasr) when not traveling, because where shortening is permissible, combining is permissible, but not vice versa, and the Hadith only mentions combining prayers.