Musannaf Ibn Abi Shayba 3:5352

Ibn Idris narrated to us, from Harun ibn Abi Maryam, from Ibn al-Aswad, that he used to pray forty rak'ahs with them, and he would perform witr with them, and he would pray twelve rak'ahs between the two tarwihat (intervals). And he would say between the two tarwihat: