Sunan an Nasai
4798
Sahih Lighairihi+2

It was narrated from Ya'qub bin Aws that:a man from among the Companions of the Prophet narrated to him that the Prophet 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

Sahih LighairihiAbu Ghuddah
SahihAl-Albani