Al-Matalib al-'Aliyah 415 — The command to follow the imam

Sayyiduna Ali bin Shayban narrates from his father (may Allah be pleased with them both) that he said: I prayed behind the Prophet (peace and blessings be upon him), and (during the prayer) a man raised his head (from bowing or prostration) before the Prophet (peace and blessings be upon him). When he (peace and blessings be upon him) completed the prayer and turned, he said: "Whoever raises his head before the imam, or puts it down (in prostration) before him, there is no prayer for him."
Imam Musaddad narrated this report in his Musnad; it contains the warning that preceding the imam invalidates the prayer or negates its reward.