It was narrated from Ya'qub bin Aws that: a man from among the Companions of the Prophet (ﷺ) told him, that when the Messenger of Allah (ﷺ) came to Makkah, in the Year of the Conquest, he 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 حَدَّثَنَا يَزِيدُ، قَالَ حَدَّثَنَا خَالِدٌ، عَنِ الْقَاسِمِ بْنِ رَبِيعَةَ، عَنْ يَعْقُوبَ بْنِ أَوْسٍ، أَنَّ رَجُلاً، مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم حَدَّثَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم لَمَّا قَدِمَ مَكَّةَ عَامَ الْفَتْحِ قَالَ " أَلاَ وَإِنَّ قَتِيلَ الْخَطَإِ الْعَمْدِ قَتِيلَ السَّوْطِ وَالْعَصَا مِنْهَا أَرْبَعُونَ فِي بُطُونِهَا أَوْلاَدُهَا " .
sunnah.com