'Fulfill' the law is a term found repeatedly in ancient Jewish writings. It is not used in the sense of 'fill up', but 'to do' and 'to keep' the law. Here are a few examples:
If the Sanhedrin gives a decision to abolish (uproot, la'akor) a law, by saying for instance, that the Torah does not include the laws of Sabbath or idolatry, the members of the court are free from a sin offering if they obey them; but if the Sanhedrin abolishes (la'akor) only one part of a law but fulfills (lekayem) the other part, they are liable. - Mishnah, Horayot 1:3
If this is how you act, you have never in your whole life fulfilled the requirement of dwelling in a sukkah! - Mishnah, Sukkot 2:7
Whoever fulfills the Torah when poor will in the end fulfill it in wealth. And whoever treats the Torah as nothing when he is wealthy in the end will treat it as nothing in poverty - Mishnah, Avot 4:9
For more, see J. B. Lightfoots Commentary on the New Testament from the Talmud and Hebraica on
Matthew 5.
Midrash
Midrash Rabbah -
Exodus 30:22
22. Another explanation of NOW THESE ARE THE ORDINANCES. Both the heathen and Israel have judges, and you do not know what difference there is between both. It can be compared to a sick man whose doctor paid him a visit and then said to the family: ' Give him to eat whatever he wants.' When he came to the other, he left word: ' Take care not to let him eat that and that thing.' When he was asked, `The first, thou didst allow to eat whatever he wishes, and the second, thou didst forbid certain things,' his reply was: `The first has no chance of recovering; for this reason did I allow him to eat what he fancies; but the second will yet live and therefore did I command strict caution in his diet.' Similarly the heathen have judges,
but neither study the Torah nor fulfil it, as it says, Wherefore I gave them also statutes that were not good, and ordinances whereby they should not live (Ezek.XX, 25),1 but with regard to the commandments it says, Which if a man do, he shall live by them (Lev. XVIII, 5).
Exodus Rabbah 40:1
R. Hoshaya said: Anyone who has knowledge but lacks the fear of sin, really has nothing, just as a carpenter who has no tools with him is not a real carpenter; because the bolts which guard learning are the fear of sin, as it says, And the fear of the Lord is its treasure-house (Isa. XXXIII, 6). R. Johanan said:
If one knows the Torah but does not fulfil it, it were better for him that he had not been born,
Numbers Rabbah 3:12
He was called Kohath for this reason: You read: If the iron be blunt (Eccl. X, 10), which means, if you perceive that the heavens have become `blunt' and refuse to send down rain, having become like iron- as you read: 'And I will make your heaven as iron'-be assured that it is as a punishment for the
non-observance of the Torah; for it is written, And one do not whet the edge (ib.), that is, because
they did not fulfil the commandments of the Torah which was given to them by God face to face-as you read, The Lord spoke with you face to face, etc. (Deut. V, 4)
Numbers Rabbah 11:1
The wise shall inherit honour (Prov. III, 35) applies to Israel who are called wise
when they fulfil the Torah and the commandments;
as it says, Observe therefore and do them; for this is your wisdom and your understanding, etc. (Deut. IV, 6)
Deuteronomy Rabbah 1:21
The Rabbis say: Moses declared before God: `Master of the Universe, just because the Gentiles have not been commanded to observe the Sabbath, wilt Thou show favour to them if they do observe it?' God replied to him: ' Do you really fear this? By your life,
even if they fulfil all the commandments in the Torah, yet will I cause them to fall before you.' Whence this? Because the text says, BEHOLD, I HAVE BEGUN TO DELIVER UP BEFORE THEE.
Deuteronomy Rabbah 7:4
TO OBSERVE TO DO ALL HIS COMMANDMENTS (XXVIII, 1). R. Simeon b. Halafta said: If one learns the words of the Torah
and does not fulfil them, his punishment is more severe than that of him who has not learnt at all.
Deuteronomy Rabbah 11:6
Halachah: When a Jew goes up to read the Law, he is not permitted to commence reading it before he has recited the blessings. First he must recite the blessings and then he reads. And thus Moses, when he had the privilege of receiving the Torah, first recited a blessing, and then he read it. R. Eleazar asked: What was the blessing which Moses recited before reading it? [It was], Blessed art Thou, O Lord, King of the Universe, who hast chosen this law and sanctified it and hast found pleasure in them
who fulfil it. He did not say, ' in them that labour at it,' nor, ' in them who meditate in it,'
but, ' in them that fulfil it,' that is to say, in them who carry out the words of the Torah.
Numbers Rabbah 10:8
For thus have the Sages said: Keep away from a small sin lest it lead you to a grievous one; run to
fulfil a small commandment, for it will lead you to an important one.
Talmud
Chagigah 4b
Samuel went and brought Moses with him, Saying to him: Perhaps, Heaven forfend,21 I am summoned to Judgment: arise with me,22 for t
here is nothing that thou hast written in the Torah, which I did not fulfil.
Arachin 22a
R. Papa said: The paying of a debt is a commandment and [minor] orphans are not obliged to
fulfil the commandment.
Mishnah
AVOT 4:9
R. JONATHAN SAID:
WHOEVER FULFILS THE TORAH OUT OF [A STATE OF] POVERTY, HIS END
[WILL BE] TO FULFIL IT OUT OF [A STATE OF] WEALTH; AND
WHOEVER DISCARDS THE TORAH OUT OF [A STATE OF] WEALTH, HIS END [WILL BE] TO DISCARD IT OUT OF [A STATE OF] POVERTY.