Muwatta Malik · Hadith 311

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

Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).