(a) Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: Whoever recites behind the imam, his prayer is not valid.
(b) If this hadith is authentic with these words, and there is doubt about that, it is to be understood as referring to reciting aloud. And Allah Almighty knows best.
Abdullah bin al-Walid al-Adani differed from him and narrated it from Sufyan from Umar bin Muhammad from Musa bin Sa'd from Zayd, without mentioning his father in its chain. Al-Bukhari said: With this chain, it is not known that some of them heard from others, and the like of it is not authentic.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2912