Must?
A person can get his feet wet and even drown because the water's deep and he cannot swim. But, on the other hand if a man is told by God to open his mouth, will not God fill it with any necessary words whether the man has read the Bible or not? Man is limited, but God is not. If we are obeying God, will not God provide all that is needed?
"And Abraham said, My son, God will provide himself a lamb for a burnt offering: so they went both of them together." Gen 22:8
"And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son." Gen 22:13
Abraham went up the mountain with his son walking in faith with God. He did not know ahead of time that the ram would be there to be sacrificed instead of his son. If we trust Him, God will always provide what is needed, even if we don't the scriptures which would support us.
No we cannot make it up and expect to be believed, but even if do quote the Bible verbatim many will not believe. Consider these verses:
"Abraham saith unto him, They have Moses and the prophets; let them hear them.
And he said, Nay, father Abraham: but if one went unto them from the dead, they will repent.
And he said unto him, If they hear not Moses and the prophets, neither will they be persuaded, though one rose from the dead." Luke 16:29-31
Well one did rise from the dead, and still many people are not persuaded. Having the scriptures on our side is necessary because they contain the Word of God, but if we cannot recall the precise verses that should not stop us from testifying as to what is written in our heart.
If a person is held at gun point and ask what he believed about Jesus, would it be right for him to deny that Jesus is risen because he cannot recall which scriptures confirm it?