Sayyiduna Abdullah bin Shaddad (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said: "Whoever is praying in congregation with an imam, the recitation of the imam is his recitation."
Likewise Ali bin al-Hasan bin Shaqiq narrated it from Ibn al-Mubarak (may Allah have mercy on him), and likewise it was reported by reliable narrators such as Sufyan bin Sa'id al-Thawri, Shu'bah bin al-Hajjaj, Mansur bin al-Mu'tamir, Sufyan bin Uyaynah, Isra'il bin Yunus, Abu Awanah, Abu al-Ahwas, Jarir bin Abd al-Hamid and others. This same narration was reported as connected (mawsul) by al-Hasan bin Umarah through Musa, and al-Hasan bin Umarah is matruk (abandoned).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2897