It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) as a marfu' report (from the Prophet (peace and blessings be upon him)): "It is as if I have been called and I have answered, and I am leaving among you the two weighty things: the Book of Allah, a rope stretched between the heaven and the earth, and my 'itrah (kin), my household. And these two will never separate until they come to me at the Pool. So see how you treat them both after me."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2666