Khusayf narrated: I asked Sa'id bin Jubayr: Which is graver (as a sin), adultery or accusing a chaste woman? He said: Adultery. I said: But Allah says: "Indeed, those who accuse chaste, unaware, believing women" [Surah al-Nur: 23]. He said: This was revealed specifically about Sayyidah Aishah.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19531