Majma al-Zawaid — Hadith 2414

Sayyiduna Abu Umamah (may Allah be pleased with him) narrates this saying of the Prophet (peace and blessings be upon him): "When the imam says the takbir, you also say the takbir; when he bows, you also bow; when he prostrates, you also prostrate; when he raises his head from bowing, you also raise yours; and when he prays sitting, you also pray sitting."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Ufayr bin Ma'dan, who is da'if (weak).