Musnad Ahmad 1153:357

Hassan narrated to us, Ibn Lahi'ah narrated to us, he said: Abu al-Aswad narrated to us, from 'Urwah, from 'Aishah, that the Messenger of Allah, peace and blessings of Allah be upon him, used to say: "Assign a portion of your prayer in your houses, and do not make them graves for yourselves."