Musnad Ahmad 403:11

Abdullah ibn Ahmad said: Abu Humayd al-Homsi, Ahmad ibn Muhammad ibn al-Mughirah ibn Sayyar, narrated to me, saying: Haywah narrated to us, saying: Baqiyyah narrated to us, from Safwan ibn Amr, from Rashid ibn Sa’d, who said: When Istakhr was conquered, a herald called out: "Behold, the Dajjal has emerged!" He said: Sa’b ibn Jaththamah met them and said: "Were it not for what you are saying, I would have informed you that I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'The Dajjal will not emerge until people become heedless of his mention, and until the Imams cease mentioning him upon the pulpits.'"