Yazid bin Hayyan said: I heard Sayyiduna Zayd bin Arqam (may Allah be pleased with him) say: One day the Prophet (peace and blessings be upon him) stood among us delivering a sermon. After praising and extolling Allah, he (peace and blessings be upon him) said: "O people, I am only a human being. Soon the messenger of my Lord will come and I will respond to him (meaning death). I am leaving among you two things: the first is the Book of Allah, in which there is guidance and light. So hold fast to the Book of Allah." Then he (peace and blessings be upon him) urged (adherence to) the Book of Allah and encouraged it. Then he said: "The second is my household; I exhort you regarding my household," three times.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20335