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And he took the cup, and gave thanks, and gave [it] to them, saying, Drink ye all of it; For this is my blood of the new testament, which is shed for many for the remission of sins. But I say unto you, I will not drink henceforth of this fruit of the vine, until that day when I drink it new with you in my Father's kingdom. Matthew 26:27-29

And he said unto them, This is my blood of the new testament, which is shed for many. Verily I say unto you, I will drink no more of the fruit of the vine, until that day that I drink it new in the kingdom of God. Mark 14:24-25

And he took the cup, and gave thanks, and said, Take this, and divide [it] among yourselves: For I say unto you, I will not drink of the fruit of the vine, until the kingdom of God shall come. Luke 22:17-18

Leaven was always symbolic of evil and therefore could not have been used to ferment the "fruit of the vine" in the ordinance of the Communion.