Abu Ghalib narrates from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) used to pray two rak'ahs sitting after the witr, reciting in them "When the earth is shaken" [Surah al-Zalzalah: 1] and "Say: O disbelievers" [Surah al-Kafirun: 1].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4823