Musannaf Ibn Abi Shayba 7:281

Mu'tamir narrated to us, from Layth, from Abu Ma'shar, from Yazid ibn Ibrahim al-Taymi, who said: If a man says, "It is upon me for the sake of Allah" or "Upon me is a Hajj," then they are the same. And if he says, "For the sake of Allah, upon me is a vow" and "Upon me," then they are the same.