Musannaf Ibn Abi Shayba 27:775

Husayn ibn Ali narrated to us, from Za'idah, from 'Asim, from Khaythamah, from al-Nu'man ibn Bashir, from the Prophet (may Allah bless him and grant him peace), who said: "The best of people are my generation, then those who follow them, then those who follow them, then those who follow them. Then a people will come whose testimonies will precede their oaths, and their oaths will precede their testimonies."