Musannaf Ibn Abi Shayba 19:665

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Hajjaj, from ‘Ata’, and from Ibn Abi Najih, from Mujahid, regarding the saying of the Exalted: "And let not pity for them make you lenient in the religion of Allah." They both said: It is not regarding killing, but rather regarding the establishment of the prescribed punishment (hadd).