Reference for Yevamot 227:12
וקסבר מפרק כלאחר יד הוא שבת דאיסור סקילה גזרו רבנן יו"ט דאיסור לאו לא גזרו ביה רבנן
If danger was involved, [the sucking should be permitted] even on the Sabbath also; and if no danger was involved, it should be forbidden even on a festival! — This can only be understood as a case where<span class="x" onmousemove="('comment',' Lit., 'not necessary (but)'. ');"><sup>50</sup></span> pain<span class="x" onmousemove="('comment',' Not danger to life. ');"><sup>51</sup></span> was involved, [Abba Saul] being of the opinion [that sucking]<span class="x" onmousemove="('comment',' From the breast. ');"><sup>52</sup></span> is an act of indirect<span class="x" onmousemove="('comment',' Or 'unusual'. [H] lit., 'as if by the back of the hand'. ');"><sup>53</sup></span> detaching.<span class="x" onmousemove="('comment',' [H] (rt. [H] in Pi'el, 'break down', 'detach') Milking an animal with one's hands is regarded as direct detaching which on the Sabbath is Pentateuchally forbidden (cf Shab. 95a); releasing the milk by sucking is an unusual, or indirect unloading and is only Rabbinically forbidden. ');"><sup>54</sup></span>