To what extent is the hadith narrated from Al-Hasan, from the Prophet (peace and blessings be upon him), authentic: "The severity and agony of death upon a believer is like the pain of three hundred sword strikes"?

The mursal (discontinued) hadith (definition) is a category of weak hadith. The book Tanbih al-Ghafilin by al-Samarqandi contains many fabricated and weak hadiths. One should turn away from it and towards other books that have focused on transmitting the authentic (sahih (definition)) and good (hasan (definition)) hadiths.