Musnad Ahmad 937:139

Muhammad ibn Ja‘far and Hajjaj narrated to us, they said: Shu‘bah narrated to us, from Abu ‘Imran, from ‘Abdullah ibn al-Samit, from Abu Dharr, who said: My beloved (Khalil), may Allah bless him and grant him peace, enjoined upon me three things: "Listen and obey, even if it be to a slave whose limbs are severed. And if you make broth, increase its water, then look to a household among your neighbors and provide them with some of it in kindness. And perform the prayer at its time. If you find that the Imam has already prayed, then you have secured your prayer; otherwise, it is a supererogatory prayer (nafilah)."