Yahya related to me from Malik that he had heard that Urwa ibnaz-Zubayr and Said ibn al-Musayyab used to pray voluntary prayerssitting. (ES)
App reference (ES): Book 8, Hadith 25 (ES)
USC-MSA web (English) reference: Book 8, Hadith 25 (ES)
Arabic reference (ES): Book 8, Hadith 314 (ES)