It is narrated from Umar bin Ata bin Abi al-Khuwar that Nafi' bin Jubayr sent him to Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) to ask him what Sayyiduna Mu'awiyah (may Allah be pleased with him) had said about something he had done. Al-Sa'ib bin Yazid said: I prayed with him in the maqsurah, then I stood up in my place to pray (a voluntary prayer), and he said: Do not do so until you have moved forward from here or have spoken (do not pray voluntary prayers until then), for the Messenger of Allah (peace and blessings be upon him) ordered us to do so. This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it!