Musnad Abu Ya'la al-Mawsili 52:1

Abu Khaythama narrated to us, Abdullah bin Yazid narrated to us, and Ahmad bin al-Dawraqi narrated to us, Abu Abd al-Rahman narrated to us, Sa'id bin Abi Ayyub narrated to me, Abu Hani' narrated to me, 'Amr bin Hurayth narrated to me, that the Messenger of Allah (peace be upon him) said: "Whatever you lighten from the work of your servant, then its reward is in your scales."