Musnad Abu Ya'la al-Mawsili 243:105

Ishaq narrated to us, Jarir narrated to us, from Sulayman al-Taymi, from Qatadah, from Yunus ibn Jubayr Abu Ghallab, from Hattan ibn Abdullah, from Abu Musa, from the Prophet (may Allah bless him and grant him peace), who said: "When he (the Imam) says the takbir, then say the takbir. And when he recites, then listen silently."