Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: The imam has been appointed only so that he may be followed. When he bows, you also bow; when he raises his head, you also raise yours. Do not get ahead of him when he bows, or raises his head, or prostrates; for if you are in that state, you will catch up with him in what he has gone ahead of you by: he will prostrate before you and raise his head before you, and so you will catch up with him.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2418