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