Where do you find in scripture that it was unleavened bread?
Matthew 26
The Last Passover
20(X)Now when evening came, Jesus was reclining at the table with the twelve disciples.
21As they were eating, He said, “(Y)Truly I say to you that one of you will betray Me.”
22Being deeply grieved, they each one began to say to Him, “Surely not I, Lord?”
23And He answered, "(Z)He who dipped his hand with Me in the bowl is the one who will betray Me.
24"The Son of Man is to go, (AA)just as it is written of Him; but woe to that man by whom the Son of Man is betrayed! (AB)It would have been good for that man if he had not been born."
25And (AC)Judas, who was betraying Him, said, “Surely it is not I, (AD)Rabbi?” Jesus said to him, “(AE)You have said it yourself.”
The Lord’s Supper Instituted
26(AF)While they were eating, Jesus took some bread, and (AG)after a blessing, He broke it and gave it to the disciples, and said, “Take, eat; this is My body.”
27And when He had taken a cup and given thanks, He gave it to them, saying, "Drink from it, all of you;
28for (AH)this is My blood of the covenant, which is poured out for (AI)many for forgiveness of sins.
29"But I say to you, I will not drink of this fruit of the vine from now on until that day when I drink it new with you in My Father’s kingdom."
30(AJ)After singing a hymn, they went out to (AK)the Mount of Olives.
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Passover Bread
The Jewish festival of Passover (Hebrew, and Yiddish: פֶּסַח, Tiberian: pɛsaħ, Israeli: Pesach, Pesah, Pesakh) is celebrated on the 14th day of the month called Nisan (Lev 23:4; Num 9:3,5, Num 28:16), first month of the Jewish year. It immediately precedes the Festival of the Unleavened Bread (חַג הַמַּצּוֹת, ħaɣ ham:asʕ

θ, Chag Hamatzot/s), a Jewish holiday which begins on the 15th day of Nisan (Lev 23:6, Num 28:17, Num 33:3) and is celebrated in the northern spring season. In 2008, it arrives at nightfall on April 19.