مُوَطَّأُ مَالِكٍ
Muwatta Malik 314
Prayer in Congregation
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عُرْوَةَ بْنَ الزُّبَيْرِ، وَسَعِيدَ بْنَ الْمُسَيَّبِ، كَانَا يُصَلِّيَانِ النَّافِلَةَ وَهُمَا مُحْتَبِيَانِ .
Yahya related to me from Malik that he had heard that Urwa ibn az-Zubayr and Said ibn al-Musayyab used to pray voluntary prayers sitting.
Arabic/English book reference: Book 8, Hadith 25