Is what is narrated about breast milk—that it is "a provision from Allah; in summer it is cool, and in winter it is warm, and these are two veins of which none of His creation knows where they are"—a sound hadith narrated from the Prophet, peace and blessings be upon him? And has it been scientifically proven that breast milk changes in temperature according to the seasons?

This hadith (definition) was not found after searching for it in the books of Sunnah (definition) and narrations. However, there is a sacred hadith (Hadith Qudsi) close in meaning to it, in which Allah Almighty says: "So I extracted for you in your mother's breast a vein that produces milk, cool in summer, warm in winter, and I extracted it for you from between skin, flesh, blood, and veins." This was narrated by Abu Nu'aym in "Hilyat al-Awliya'" from Muhammad ibn Ka'b al-Qurazi, who was a Tabi'i (follower) and not a Companion.

Previous scriptures have been subjected to alteration, so what is narrated from them cannot be trusted. And this narration from Muhammad ibn Ka'b al-Qurazi is not authenticated because its chain of narration includes Musa ibn Ubaydah ar-Rabadhi, who is weak and whose narrations are rejected. As for the medical aspect, that should be referred to physicians.