Musnad Abu Ya'la al-Mawsili 6:71

'Amr ibn Muhammad al-Naqid narrated to us, Abu Mu'awiyah Muhammad ibn Khazim narrated to us, Isma'il ibn Abi Khalid narrated to us, from Qays, who said: Sa'd ibn Abi Waqqas led us in prayer, and he stood up during the two rak'ahs (without sitting for the first tashahhud). We glorified Allah (said SubhanAllah) to him, but he completed his standing. He said: "Did you think I would sit? I only did as I saw the Messenger of Allah (ﷺ) do." Abu 'Uthman 'Amr ibn Muhammad al-Naqid said: We have not heard anyone attribute this (to the Prophet) except Abu Mu'awiyah.