Musnad Abu Ya'la al-Mawsili 151:97

'Abdullah ibn 'Umar ibn Aban narrated to us, Abu al-Ahwas narrated to us, from Abu Ishaq, from Abu 'Ubaydah, from 'Abdullah ibn Mas'ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Show mercy to those who are on the earth, and He who is in the heaven will show mercy to you."