It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Allah Almighty has not sent down any disease without also sending down its cure, and in the milk of cows there is a cure for every disease." This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it. Abu Abd al-Rahman al-Sulami and Tariq bin Shihab have both narrated it from Abdullah bin Mas'ud. As for the hadith of Abu Abd al-Rahman al-Sulami:

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him); he said that the Messenger of Allah (peace and blessings be upon him) said: "Allah Almighty has not sent down any disease without sending down its cure along with it; whoever came to know it knows it, and whoever did not know it remains ignorant of it."

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates it from the Prophet (peace and blessings be upon him) as a marfu' report, that he (peace and blessings be upon him) said: "Indeed, Allah Almighty has not sent down any disease without sending down its cure, except old age. So hold fast to (drinking) the milk of cows, for they graze from every kind of tree and herb."

It is narrated from Urwah bin al-Zubayr that he said to Sayyidah Aishah (may Allah be pleased with her): You learned the Sunnahs from the Messenger of Allah (peace and blessings be upon him), and poetry and the Arabic language from the Arabs; but from whom did you acquire the knowledge of medicine? She replied: the Messenger of Allah (peace and blessings be upon him) was often ill, and the physicians of the Arabs used to come to him, so I would learn (methods of treatment) from them. This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Safwan bin Assal al-Muradi (may Allah be pleased with him); he said that the people said: O Messenger of Allah, shall we seek medical treatment? He (peace and blessings be upon him) said: "Know that Allah Almighty has not sent down any disease without also sending down its medicine, except for one disease." They said: What is it? He (peace and blessings be upon him) said: "Al-haram: old age." This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.