Abu Qilabah narrates from Sayyiduna Anas (may Allah be pleased with him): "When you marry a virgin while having a previously married wife, stay with her seven days; and when you marry a previously married woman while having a virgin wife, stay with her three days." If I were to say that he narrated it as marfu' (from the Prophet), I would be speaking the truth, because he calls it the Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسم والنشوز / 14761