סימן פ סעיף יג

לֹא תְקַלַּח אִשָּׁה חָלָב מִדַּדֶּיהָ לְתוֹךְ הַכּוֹס אוֹ לְתוֹךְ הַקְּדֵרָה וְתֵינִיק אֶת בְּנָהּ. אֲבָל מֻתֶּרֶת לְקַלֵּחַ מֵהֶחָלָב כְּדֵי שֶׁיֹּאחֵז הַתִינוֹק אֶת הַדַּד וְיִינָק. וְאָסוּר לְהַתִּיז מֵחֲלָבָהּ עַל אֵיזֶה דָּבָר לִרְפוּאָה בְּמָקוֹם שֶׁאֵין בּוֹ סַכָּנָה וְלֹא צַעֲרָא יְתֵרָה.

A woman may not squeeze milk from her breasts⁵¹According to most Rishonim, milking is מְפַרֵק (mefareik), a toldah of dosh, threshing. into a cup⁵²Milking into an empty cup is a deoraisa. Milking into food is a shevus (Rabbinic prohibition). or pot⁵³You are not allowed to milk into an empty vessel even with the intention to discard it. [see Chazon Ish 55: 6—(also see footnote 50)] and feed her child with it,⁵⁴Even though nursing is mefareik, it is permitted to nurse a baby on Shabbos, for it is considered an irregular form of mefareik. See Biur Halachah 328: 34.
The Chelkas Yoav (Resp. 9) says that a baby may nurse on Shabbos, for since this is its way of eating, it is not considered mefareik. But for an adult, it would be considered mefareik. but she may squeeze out a little milk⁵⁵Into the mouth of the baby. (Shibolei Haleket Ch. 123, Chayei Adam 69: 10, Mishnah Berurak 328: 112) to facilitate the baby's nursing. It is forbidden to sprinkle some of her milk on something for the sake of a remedy where there is no danger involved and no intense pain.⁵⁶In case the mother is in pain, she may express the milk with a manual breast pump, even though the milk goes into the pump before being discarded. [see Avnei Neizer (Orach Chaim Vol. I Resp. 47)]

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)