Musnad Abu Ya'la al-Mawsili 158:1

Al-Harith ibn Surayj narrated to us, Sufyan narrated to us, from Hisham ibn Hujayr, from Tawus, from Abu Hurayrah, attributing it to the Prophet (may Allah bless him and grant him peace)—and he mentioned it once without attributing it—that Sulayman (peace be upon him) swore an oath that he would go around to ninety women tonight, each of whom would give birth to a boy who would fight in the way of Allah. The angel, or his companion, said to him: "Say, 'If Allah wills.'" But he forgot. So he went around to them, and no woman brought forth anything, except for one of them who brought forth half a boy. The Prophet (may Allah bless him and grant him peace) said: "If Sulayman had made an exception (by saying 'If Allah wills'), he would not have broken his oath, and it would have been a means of attaining his need."