Musnad Ahmad 30:1542

Hassan ibn Musa narrated to us, Abd al-Rahman ibn Abd Allah ibn Dinar al-Madini narrated to us, from Zayd ibn Aslam, from Ata ibn Yasar, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "They will lead you in prayer; if they are correct, it is for you and for them, and if they err, it is for you and against them."