Qabus bin Abi Zabyan said that his father said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) passed by the Mosque of the Prophet and prayed one rak'ah, then went on. A man caught up with him from behind and said: "O Commander of the Believers! You have prayed only one rak'ah." He (may Allah be pleased with him) said: "This is a voluntary prayer; whoever wishes may pray one rak'ah, and whoever wishes may add to it or reduce it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4781