Musnad Ahmad 962:110

'Abd al-Samad narrated to us, my father narrated to me, 'Abd al-'Aziz—meaning Ibn Suhayb—narrated to us, from Abu Ghalib, from Abu Umamah: "That the Prophet, peace and blessings of Allah be upon him, used to pray them [two rak'ahs] after the Witr while he was sitting. He would recite in them: 'When the earth is shaken' [Surah Az-Zalzalah] and 'Say, O disbelievers' [Surah Al-Kafirun]."