The word hadith refers to "speech," meaning, in this case, both collective and individual reports of what the Prophet said and did. Through these words is known the Sunna, which is the model of behavior that all Muslims should imitate. The earliest written collections of hadith date to the last third of the eighth century CE. The Six Books, which make up the most highly respected Sunni collections, are those of al-Bukhari (d.870),Muslim (d.875),Abu Dawud (d. 889), al-Tirmidhi (d. 892), al-Nasa'i (d. c. 915), and Ibn Maja (d. c. 887). Four collections enjoy almost as much prestige among the Twelver Shi'is, mainly those of al-Kulayni (d. c. 940) and Ibn Babawayh (d. 991) and two books by al-Tusi (d. c. 1067). Other sects have their own collections of hadith.