Musnad Abu Ya'la al-Mawsili 158:3

Ishaq bin Abi Isra'il and Abu Bakr bin Zanjawayh narrated to us, they said: Abd al-Razzaq narrated to us, Ma'mar informed us, from Ibn Tawus, from his father, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever swears an oath and says 'If Allah wills' (Insha'Allah), he has not broken his oath."