Musannaf Ibn Abi Shayba 3:5183

Hatim ibn Isma'il narrated to us, from Ja'far, from his father, who said: Al-Hasan ibn Ali and Al-Husayn used to pray behind Marwan. It was said to him: "Did your father not pray when he returned home?" He said: "He would say: 'No, by Allah, they would not add anything to the prayer of the Imams.'"