Musnad Ahmad 32:475

Abu al-Nadr narrated to us, al-Mubarak narrated to us, Humayd al-Tawil narrated to me, from Anas ibn Malik, who said: When the Messenger of Allah, peace and blessings of Allah be upon him, passed away, it was said: "There is a man who digs a *lahd* (a niche in the side of the grave) and another who digs a *shaqq* (a trench in the center of the grave)." They said: "We shall seek the guidance of our Lord, and we shall send for both of them; whichever of them arrives first, we shall leave the matter to him." So they sent for them, and the one who dug the *lahd* arrived first, so they dug the *lahd* for him.