Reference: Mishnah>> Seder Nashim>> Shabbat>> Chapter 19>> Verse 2
Online Read: http://www.sefaria.org/Mishnah_Shabbat.19.2?lang=en
Original Text:One may do whatever is necessary for a circumcision on Shabbat. One may circumcise, and tear [the membrane], and suck [the blood], and put a bandage and cumin on it. If he did not grind it [the cumin] on Shabbat eve he may chew it with his teeth and put it on. If he did not mix wine and oil on Shabbat eve he may put this by itself and this by itself. One may not prepare a dressing [on Shabbat] but he may wrap a rag around it. If he did not prepare it on Shabbat eve he may wrap it round his finger and bring [it] – even from another courtyard.
Talmud>> Shabbat 133b:14-15
We learned in the mishna that one sucks blood from the wound after the circumcision was performed on Shabbat. Rav Pappa said: A craftsman who does not suck the blood after every circumcision is a danger to the child undergoing circumcision, and we remove him from his position as circumcisor. The Gemara comments: This is obvious. Given that one desecrates Shabbat to suck the blood, which involves performance of a prohibited labor, obviously, failure to do so poses a danger. Desecration of Shabbat would not be permitted if it was not a life-threatening situation. The Gemara answers: This is not an absolute proof. Lest you say that this blood is collected and contained in place, and one who removes that which is pooled in its place does not perform the subcategory of the prohibited labor of threshing on Shabbat, i.e., drawing blood; that is the reason sucking the blood is permitted, not due to any danger involved in failing to do so. Therefore, the mishna teaches us that this blood is attached and flowing and not merely pooled. One who draws it out performs an act that is generally prohibited by Torah law on Shabbat, and it is nonetheless permitted due to danger to the child.
Mishnah Torah>> Circumcision>> Chapter 2
2. How is circumcision performed? The entire foreskin which covers the glans is cut, so that the whole of the glans is exposed. Then the thin layer of skin (mucous membrane) beneath the foreskin in divided with the nail and turned back, till the flesh of the glans is completely exposed. The wound is then sucked till the blood has been drawn from parts remote (from the surface), thus obviating danger (to the child’s health). The operator who omits to do so is removed from practice. After this has been done, a plaster, bandage, or similar dressing is applied………….
6. All the requisites that belong to the act of circumcision are done on the Sabbath. The foreskin is removed, the mucous lamella (inner layer of skin) retracted, the blood drawn by suction.