Musnad Ahmad 340:6

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Muhammad ibn 'Amr ibn 'Ata' narrated to me, from Nu'aym ibn Mujmir, from Rabi'ah ibn Ka'b, who said: I used to serve the Messenger of Allah (peace and blessings of Allah be upon him) and attend to his needs throughout my day, until the Messenger of Allah (peace and blessings of Allah be upon him) would pray the final 'Isha' prayer. I would then sit at his door. When he entered his house, I would say: "Perhaps a need may arise for the Messenger of Allah (peace and blessings of Allah be upon him)." I would not cease to hear the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Glory be to Allah, glory be to Allah, glory be to Allah and by His praise," until I grew weary and turned back, or my eyes overcame me and I slept. He said: So he said to me one day, because of what he saw of my readiness for him and my service to him: "Ask me, O Rabi'ah, and I shall give you." He said: I said: "I will consider my affair, O Messenger of Allah, then I will inform you of that." He said: So I reflected within myself and realized that the world is cut off and vanishing, and that I have in it a provision that will suffice me and come to me. He said: So I said: "I will ask the Messenger of Allah (peace and blessings of Allah be upon him) for my Hereafter, for he holds a position with Allah, the Mighty and Majestic." He said: So I came, and he said: "What have you decided, O Rabi'ah?" He said: I said: "Yes, O Messenger of Allah. I ask you to intercede for me with your Lord so that He may free me from the Fire." He said: So he said: "Who commanded you to do this, O Rabi'ah?" He said: I said: "No, by the One who sent you with the truth, no one commanded me to do it. But when you said 'Ask me and I shall give you,' and you hold the position with Allah that you hold, I considered my affair and realized that the world is cut off and vanishing, and that I have in it a provision that will come to me. So I said: I will ask the Messenger of Allah (peace and blessings of Allah be upon him) for my Hereafter." He said: The Messenger of Allah (peace and blessings of Allah be upon him) remained silent for a long time, then said to me: "I will do so, so help me against your own self with an abundance of prostration."