Sunan an-Nasa'i 3856
It was narrated from Abu Hurairah, who attributed it to the Prophet:
"Sulaiman said: 'I will certainly go around to ninety women tonight, each of whom will bear a child who will fight in the cause of Allah.' It was said to him: 'Say: If Allah wills' but he did not say it. He went around to them but none of them bore a child except for one woman who bore half a person." The Messenger of Allah said: "If he had said: 'If Allah wills,' he would not have broken his vow, and this would have been a means to help him to get what he wanted."
أَخْبَرَنَا الْعَبَّاسُ بْنُ عَبْدِ الْعَظِيمِ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَنْبَأَنَا مَعْمَرٌ، عَنِ ابْنِ طَاوُسٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، رَفَعَهُ ‏"‏ قَالَ سُلَيْمَانُ لأَطُوفَنَّ اللَّيْلَةَ عَلَى تِسْعِينَ امْرَأَةً تَلِدُ كُلُّ امْرَأَةٍ مِنْهُنَّ غُلاَمًا يُقَاتِلُ فِي سَبِيلِ اللَّهِ فَقِيلَ لَهُ قُلْ إِنْ شَاءَ اللَّهُ ‏.‏ فَلَمْ يَقُلْ فَطَافَ بِهِنَّ فَلَمْ تَلِدْ مِنْهُنَّ إِلاَّ امْرَأَةٌ وَاحِدَةٌ نِصْفَ إِنْسَانٍ ‏"‏ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لَوْ قَالَ إِنْ شَاءَ اللَّهُ لَمْ يَحْنَثْ وَكَانَ دَرَكًا لِحَاجَتِهِ ‏"‏ ‏.‏