The oldest Greek manuscript we have of any significant part of the Gospel of Matthew is probably Papyrus 64/67 (?⁶⁴). Regarding Papyrus 64, there are 7 places where the Byzantine Robinson-Pierpont (RP) text and/or the Textus Receptus (TR) disagree with the Critical Text (CT), and where P64 is also extant:

5:20, CT and P64 read υμων η δικαιοσυνη, RP has η δικαιοσυνη υμων

5:22, CT and P64 read αυτου, RP reads αυτου εικη

5:25, CT and P64 read ο κριτης, RP reads ο κριτης σε παραδω

5:27, CT and P64 and RP read ερρεθη, TR reads ερρεθη τοις αρξαιοις (difference between TR and Robinson-Pierpont)

26:8, CT and P64 read μαθηται, RP reads μαθηται αυτου

26:22, CT reads εις εκαστος, RP reads εκαστος αυτων, but P64 omits both of these

26:23, CT and P64 read μετ εμου την χειρα εν τω τρυβλιω, RP reads μετ εμου εν τω τρυβλιω την χειρα

So, P64 agrees with CT 5 out of 7, agrees with CT and RP against TR 1 out of 7, and against both CT and RP 1 out of 7.

The only really significant variant is 5:22. CT and P64 say “angry with his brother,” and RP text says “angry with his brother without a cause”

5:27, CT and P64 and RP say “You have heard that it was said,” and TR says, “You have heard that it was said to the people long ago.”

26:22, CT says, “they began each one to say to him; RP says “they began each one of them to say to him,’ and P64 says “they began to say.”