Musnad Ahmad 30:969

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ibn Tawus, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever swears an oath and says, 'If Allah wills' (Insha'Allah), he has not broken his oath." 'Abd al-Razzaq said: And he—meaning Ma'mar—abbreviated it.