What is meant by "sins" in the Hadith: "May Allah curse the innovators in Madinah"?

The sins that necessitate the curse mentioned in the noble Hadith (He who innovates therein [in Medina] or gives refuge to an innovator) refer to committing a reprehensible act or an innovation (bid'ah) in religion, or protecting and supporting those who do so. Ibn Hajar holds that "al-hadath" (innovation/event) and "al-muhdith" (innovator) include injustice and the unjust person, and that innovation in Medina is among the major sins. The meaning of the curse of the angels and people is an exaggeration of being distanced from Allah's mercy. The intended meaning of the curse here is the punishment deserved for the sin, and it is not like the curse on a disbeliever.