Musannaf Ibn Abi Shayba 3:752

Abu Bakr narrated to us, he said: Rawh ibn Ubadah narrated to us, from Hammad ibn Salamah, from Hisham ibn Urwah, from his father, who said: "O my son, when I say the taslim, I sit and perform tasbih and takbir. So whoever has any part of his prayer remaining, let him stand and complete it."