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