Musnad Ahmad 30:835

Safwan narrated to us, Ibn ‘Ajlan informed us, from al-Qa‘qa‘ ibn Hakim, from Abu Salih, from Abu Hurayrah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The martyr does not feel the pain of being killed, except as one of you feels the sting of a pinch."