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