Aayesha Radiyallahu 'Anha reports:
"Rasulullah Sallallahu 'Alayhi Wasallam performed most of his (nafl) salaah in a sitting posture before he passed away". (IT)
English reference (IT): Book 39, Hadith 266 (IT)
Arabic reference (IT): Book 40, Hadith 282 (IT)