Musannaf Ibn Abi Shayba 3:4358

Waki’ narrated to us, from Shu’bah, from Abu Jamrah, from Ibn Abbas and ‘A’idh ibn ‘Amr, they both said: If you perform Witr at the beginning of the night, then do not perform Witr at the end of it. And if you perform Witr at the end of the night, then do not perform Witr at the beginning of it.