The first four Khalifs were called the Khulafa-i-Rashidin that is, those who could guide others aright. They had been friends and Companions of the Prophet, and the Faithful could always appeal to them in cases of doubt. The Prophet had declared that Islam must be written in the hearts of men. There was therefore an unwillingness to commit his sayings to writing. They were handed down by word of mouth. As no argument was so effectual in a dispute as "a saying" of the Prophet, the door was opened by which spurious Traditions could be palmed off on the Faithful. To prevent this, a number of strict rules were framed, at the head of which stands the Prophet's saying, itself a Tradition: "Convey to other persons none of my words except those which ye know of a surety. Verily, he who purposely represents my {67} words wrongly will find a place for himself nowhere but in fire." To enforce this rule, it was laid down that the relator of a Tradition must also repeat its "Isnad," or chain of authorities, as: "I heard from such an one, who heard from such an one," and so on, until the chain reaches the Prophet himself. Each person, too, in this "Isnad," must have been well known for his good character and retentive memory. This failed, however, to prevent a vast number of manifestly false Traditions becoming current; so men set themselves to the work of collecting and sifting the great mass of Tradition that in the second century of Islam had begun to work untold evil. These men are called "Muhadisin," or "collectors of Tradition." The Sunnis and the Wahhabis recognise six such men, and their collections are known as the "Sihah-Sittah," or six correct books. They are the following:--