(a) Azraq bin Qays narrates that our imam, Abu Rimthah (may Allah be pleased with him), led us in prayer and said: I prayed with the Prophet (peace and blessings be upon him) in this same way. Abu Bakr and Umar (may Allah be pleased with them both) were standing in the first row on his right. There was another man as well who had joined in the opening takbir. When the Prophet (peace and blessings be upon him) had finished the prayer, he gave the salam to his right and to his left until we saw the whiteness of his cheeks; then he stood up, just as I (Abu Rimthah) stood up. Then the man who had joined in the opening takbir stood up and began another (voluntary) prayer, so Umar (may Allah be pleased with him) rushed towards him, seized him by the shoulders, shook him and said: Sit down, for the People of the Book were destroyed only because they made no break between their obligatory and voluntary prayers.
The Prophet (peace and blessings be upon him) raised his gaze, looked, and said: "O son of al-Khattab! Allah Almighty has granted you the ability to say what is right."
(b) Imam al-Bayhaqi (may Allah have mercy on him) says: If this hadith is established, then the imam and the follower have the same ruling in this matter. Likewise is the hadith of Abu Hurayrah (may Allah be pleased with him); and in this chapter there is a hadith more authentic than all that we have mentioned.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3046