Bukhari

Hadith Commentary:

Yes, old age and death are two such ailments for which no cure has been sent down.
The word "anzala" (sent down) subtly alludes to the fact that rain, which descends from the sky, also gives rise to many disease-causing germs, and the effects that repel these are also continually sent down. Truly has Allah said: "And We made from water every living thing" (: Al-Anbiya: 30).

Hadith Commentary:
(1)
In one narration it is stated: “Seek treatment for illness, but do not use unlawful things as medicine.”
(Sunan Abi Dawud, Book of Medicine, Hadith: 3874)
In reality, Imam al-Bukhari rahimahullah intends to refute those Sufis whose position is that a person only attains the rank of sainthood (wilayah) when, upon falling ill, he does not seek treatment but rather remains content with the illness itself. However, seeking treatment is Sunnah, as is explicitly mentioned in the cited hadith, but in this matter, unlawful things should not be used for treatment.

We observe that sometimes a patient does not recover through the use of medicine; the reason for this is the lack of correct diagnosis, correct prescription, or the medicine itself, as is mentioned in a hadith in which the Messenger of Allah sallallahu alayhi wa sallam said:
“There is a remedy for every disease.
When the remedy strikes the disease precisely, by Allah’s permission, the patient is cured.”
(Sahih Muslim, Book of Peace, Hadith: 5741 (2204)) (1)

However, when death approaches or old age begins to set in, then there is no cure for that.
And Allah knows best.