It was narrated from Ya'qub bin Aws that: a man from among the Companions of the Prophet (ﷺ) narrated to him that the Prophet (ﷺ) entered Makkah during the Year of the Conquest, and said: 'Indeed, accidental killing on purpose, is killing with a whip or stick, for which forty (she-camels) with their young in their wombs."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ بَزِيعٍ، قَالَ أَنْبَأَنَا يَزِيدُ، عَنْ خَالِدٍ، عَنِ الْقَاسِمِ بْنِ رَبِيعَةَ، عَنْ يَعْقُوبَ بْنِ أَوْسٍ، أَنَّ رَجُلاً، مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم حَدَّثَهُ أَنَّ النَّبِيَّ صلى الله عليه وسلم دَخَلَ مَكَّةَ عَامَ الْفَتْحِ قَالَ " أَلاَ وَإِنَّ قَتِيلَ الْخَطَإِ الْعَمْدِ قَتِيلَ السَّوْطِ وَالْعَصَا مِنْهَا أَرْبَعُونَ فِي بُطُونِهَا أَوْلاَدُهَا " .
sunnah.com