Sayyiduna Zayd bin Arqam (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "I will go ahead of you (to the Pool), and you will come to me at the Pool. Its width is from San'a' to Busra, and in it are cups of gold and silver as numerous as the stars. So see how you treat the two weighty things (al-thaqalayn) after me." A man stood up and said: O Messenger of Allah, what are the two weighty things? The Messenger of Allah (peace and blessings be upon him) said: "The greater is the Book of Allah, a rope one end of which is in the Hand of Allah and the other end in your hands; so hold fast to it and you will neither stray nor go astray. And the lesser is my 'itrah (kin). These two will never separate until they come to me at the Pool, and I have asked my Lord for this for them. So do not go ahead of them, or you will perish, and do not try to teach them, for they know more than you."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2668