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". (PT)
English reference (PT): Book 42, Hadith 284 (PT)
Arabic reference (PT): Book 43, Hadith 301 (PT)