Musnad Ahmad 27:1515

Ya'qub narrated to us, my father narrated to us, from Salih, Nafi' narrated to us, that Abdullah ibn Umar said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "While three men were walking, rain overtook them, so they took shelter in a cave in a mountain. While they were in it, a rock fell from the mountain and closed it upon them." He then mentioned the rest of the hadith, similar in meaning.