Musannaf Ibn Abi Shayba 7:39

Waki' narrated to us, from Isma'il ibn Abi Rafi', from Khalid ibn Yazid, from 'Uqbah ibn 'Amir (may Allah be pleased with him), who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever makes a vow and does not name it, then upon him is the expiation of an oath."