Ummi Salaah Radiyallahu 'Anha reports:
"I did not observe Rasulullah Sallallahu 'Alayhi Wasallam fast for two consecutive months, besides the month of Sha'baan and Ramadhaan". (IT)
English reference (IT): Book 42, Hadith 284 (IT)
Arabic reference (IT): Book 43, Hadith 301 (IT)