Musnad Ahmad 27:1585

Sulayman ibn Dawud narrated to us, Sa'id ibn 'Abd al-Rahman—meaning al-Jumahi—narrated to us, from 'Ubayd Allah ibn 'Umar, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Pray in your houses, and do not take them as graves."