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