'A slave (of Allah) shall not believe until he believes in Al-Qadar, its good and its bad, such that he knows that what struck him would not have missed him, and that what missed him would not have struck him."
Jami` at-Tirmidhi Book 32, Hadith 12
← Previous Hadith "The Messenger of Allah (s.a.w) stood among us and said: 'One thing does not infect another.' So a Bedouin said: 'O Messenger of Allah! If a camel gets mangy glands and we leave it at the resting place of camels, then all of the camels get mange?' The Messenger of Allah (s.a.w) said: 'Who caused the first to get mange? There is no 'Adwa nor safar. Allah created every soul, so he wrote its life, its provision, and its afflictions.'"

Note: The majority of scholars interpret this to mean that these things in and of themselves do not transmit or cause harm through supernatural or hidden means but that Allah is ultimately in control and any fearful superstition around these is false.

Jami` at-Tirmidhi Book 32, Hadith 11 Next Hadith → 'A slave (of Allah) shall not believe until he believes in four: The testimony of La Ilaha Illallah, and that I am the Messenger of Allah whom He sent with the Truth, and he believes in the death, and he believes in the Resurrection after death, and he believes in Al-Qadar."

Another chain reports a similar narration.

Jami` at-Tirmidhi Book 32, Hadith 13