Language:
كتاب الطب
76
Medicine
(19)
Chapter: Leprosy
(19)
باب الْجُذَامِ
Sahih al-Bukhari 5707

Narrated Abu Huraira:

Allah's Messenger (ﷺ) said, "(There is) no 'Adwa (no contagious disease is conveyed without Allah's permission). nor is there any bad omen (from birds), nor is there any Hamah, nor is there any bad omen in the month of Safar, and one should run away from the leper as one runs away from a lion."

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.

وَقَالَ عَفَّانُ حَدَّثَنَا سَلِيمُ بْنُ حَيَّانَ، حَدَّثَنَا سَعِيدُ بْنُ مِينَاءَ، قَالَ سَمِعْتُ أَبَا هُرَيْرَةَ، يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ لاَ عَدْوَى وَلاَ طِيَرَةَ وَلاَ هَامَةَ وَلاَ صَفَرَ، وَفِرَّ مِنَ الْمَجْذُومِ كَمَا تَفِرُّ مِنَ الأَسَدِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 5707
In-book reference : Book 76, Hadith 27
USC-MSA web (English) reference : Vol. 7, Book 71, Hadith 608
  (deprecated numbering scheme)