Narrated Abu Bakr: The Prophet (ﷺ) said, "Time has come back to its original state which it had when Allah created the Heavens and the Earth; the year is twelve months, four of which are sacred. Three of them are in succession; Dhul-Qa'da, Dhul-Hijja and Al-Muharram, and (the fourth being) Rajab Mudar (named after the tribe of Mudar as they used to respect this month) which stands between Jumad (ath-thani) and Sha'ban."
The chain is ordered from the source of the saying to the compiler: 1 = the speaker/first transmitter, the following links those who passed it on in turn, and at the bottom al-Bukhari who compiled the work. Click a narrator to list every hadith in which he appears; the number in parentheses is how many hadiths across the collections on this site the narrator appears in.