Is the Hadith, "If the heavens and all therein, and the earth and all thereon, were to gather together to harm you with something, they would not harm you except with something Allah has already written for you; and if they were to gather together to benefit you with something, they would not benefit you except with something Allah has already written for you," authentic or weak?

The Prophet's hadith, "Know that if the nation were to gather to benefit you with something...", is a good and authentic hadith (hasan sahih). It was authenticated by At-Tirmidhi, Ibn Rajab, Al-Albani, and others.

Shaykh Ibn Uthaymeen explains this hadith by stating that people benefiting or harming each other only occurs through what Allah has decreed. He adds that this faith drives the believer to attach themselves to Allah, rely on Him, and not be concerned with anyone else, because everything that befalls them, whether benefit or harm, is by Allah's divine decree and destiny.