Imam al-Bukhari (810–870 CE) employed rigorous criteria for accepting a hadith. He required that the chain of narrators (Isnad) be continuous and that each narrator be of impeccable character and memory. Out of approximately 600,000 hadiths he memorized, he selected only 7,275 (including repetitions). When excluding repetitions, the number drops to around 2,600 unique hadiths.