Musannaf Ibn Abi Shayba 3:4348

Hushaym narrated to us, he said: Sulayman al-Taymi informed us, from Abu Mijlaz, from Ibn Abbas, that he used to say: If a man performs Witr at the beginning of the night, then stands at the end of the night, he should make his Witr even with one Rak'ah, then he should pray, then he should perform Witr at the end of his prayer.