Musannaf Ibn Abi Shayba 19:663

Abu Bakr narrated to us, he said: Waki’ narrated to us, from ‘Imran ibn Hudayr, from Abu Mijlaz, regarding the saying of the Exalted: "And let not pity for them make you lenient in the religion of Allah." He said: It is the establishing of the prescribed punishments (hudud) when they are brought before the authority.