The Book of Enoch

Hebrews 11:5
By faith Enoch was translated, that he should not see death, and was not found, because God had translated him: For before his translation he had this testimony, that he pleased God.

Jude 1:14
And Enoch also, the seuenth from Adam, prophesied of these, saying, Behold, the Lord commeth with ten thousands of his Saints,

Jude 1:15
To execute iudgement vpon all, and to conuince all that are vngodly among them, of all their vngodly deeds which they haue vngodly committed, and of all their heard speaches, which vngodly sinners haue spoken against him.

The quote which Enoch makes in Jude 14 is found within the early portions of The Book of Enoch... Therefore we may conclude that the disciples of Jesus The Christ studied The Book of Enoch. This is the most common explanation to how they were able to quote Enoch.