Musnad Ahmad 30:597

'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: "Sulayman ibn Dawud said: 'I will certainly go around to one hundred women tonight, and each woman among them will give birth to a boy who will fight in the way of Allah.'" He said: "And he forgot to say: 'If Allah wills.' So he went around to them." He said: "And not one woman among them gave birth, except for one who gave birth to half a human." The Messenger of Allah (peace and blessings of Allah be upon him) said: "If he had said: 'If Allah wills,' he would not have broken his oath, and it would have been a means of attaining his need."