Imam al-Bayhaqi (may Allah have mercy on him) said: It has been narrated to us from Sa'id bin al-Musayyib, Ata bin Abi Rabah, al-Hasan al-Basri, Muhammad bin Sirin and Abu Qilabah, and from Qatadah, that Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said: "Whatever you catch with the imam is the first part of your prayer, and make up whatever of the Qur'an (recitation) he got ahead of you with." Qatadah also narrated from Sa'id bin al-Musayyib the like of the saying of Sayyiduna Ali bin Abi Talib (may Allah be pleased with him).
Although this is mursal (with a gap in the chain) from Ali, it is a corroborating report (shahid) for the narration of al-Harith from Ali (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3633