Mus'ab bin Sa'd (may Allah have mercy on him) said that it was said to Sa'd (may Allah be pleased with him): "Do you pray one rak'ah of witr?" He said: "Yes. Seven rak'ahs of witr are dearer to me than five, five are dearer to me than three, and three are dearer to me than one, but I make things light for myself."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4784