Musnad Abu Ya'la al-Mawsili 152:35

Al-Abbas narrated to us, Abu al-Ahwas narrated to us, Mansur ibn al-Mu'tamir narrated to us, from Ibrahim al-Nakha'i, from Ubaydah al-Salmani, from Abdullah ibn Mas'ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of my nation are the generation that are next to me, then those who are next to them, then those who are next to them. Then there will come a people whose testimony will precede their oath, and their oath will precede their testimony." Ibrahim said: We used to be forbidden from swearing by the covenant and the testimonies.