Musnad Ahmad 254:1

Abu al-Nadr narrated to us, al-Hakam ibn Fudayl narrated to us, Ya'la ibn 'Ata' narrated to us, from 'Ubayd ibn Jubayr, from Abu Muwayhibah, the freed slave of the Messenger of Allah (peace and blessings of Allah be upon him), who said: The Messenger of Allah (peace and blessings of Allah be upon him) was commanded to pray for the people of al-Baqi', so the Messenger of Allah (peace and blessings of Allah be upon him) prayed for them one night three times. When the second night came, he said: "O Abu Muwayhibah, saddle my mount for me." He said: So he rode and I walked, until he reached them. He dismounted from his mount, and I held the mount. He stood over them—or he said: He stood by them—and said: "May what you are in be pleasant for you, in contrast to what the people are in. The tribulations have come like pieces of the night, one following the other; the latter is more severe than the former. So may what you are in be pleasant for you." Then he returned and said: "O Abu Muwayhibah, I have been given—or he said: I have been given the choice of—the keys to what will be opened for my nation after me, and Paradise, or meeting my Lord." I said: "May my father and mother be sacrificed for you, O Messenger of Allah, so inform us." He said: "For it to return upon its heels as Allah wills, so I have chosen the meeting of my Lord, the Mighty and Majestic." He did not remain after that except for seven or eight [days] until he was taken [in death], peace and blessings of Allah be upon him. Abu al-Nadr said once: "Return upon its heels."