Did you start? Finish! / who starts a good deed should finish it
If you have already begun the task, bring it to completion. You shall not leave something you started unfinished! See also: "One who begins a mitzvah is told: Finish."
("All are in need of mercy, even Abraham, whose mercy flows from the world, in his path was in need of mercy, as it was said, "And he showed mercy to my master Abraham, and you began to prosper" - at the beginning of Rava (Vilna), Parashah Chayyeh Sarai, Parashah 6)
Translation
Expression (Hebrew)
הִתְחַלְתָּ? גָּמוּר!
Definition (Hebrew)
אִם כְּבָר הִתְחַלְתָּ בַּמְּלָאכָה, הָבֵא אוֹתָהּ לִידֵי סִיּוּם. אַתְּ תַּשְׁאִיר דָּבָר שֶׁהִתְחַלְתָּ בִּלְתִּי גָּמוּר ! רְאֵה גַּם:"הַמַּתְחִיל בְּמִצְוָה אוֹמְרִים לוֹ: גָּמוּר".
("הַכֹּל צְרִיכִין לְחֶסֶד אֲפִלּוּ אַבְרָהָם שֶׁהַחֶסֶד מִתְגַּלְגֵּל בָּעוֹלָם בִּשְׁבִילוֹ נִצְרָךְ לְחֶסֶד, שֶׁנֶּאֱמַר וַעֲשֵׂה חֶסֶד עִם אֲדֹנִי אַבְרָהָם הִתְחַלְתָּ גְּמֹר"- בְּרֵאשִׁית רַבָּה (וִילְנָא) פָּרָשַׁת חַיֵּי שָׂרָה פָּרָשָׁה ס)