Sahih Muslim Book - 17, Hadith - 3626

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا هُشَيْمٌ عَنْ خَالِدٍ عَنْ أَبِى قِلاَبَةَ عَنْ أَنَسِ بْنِ مَالِكٍ قَالَ إِذَا تَزَوَّجَ الْبِكْرَ عَلَى الثَّيِّبِ أَقَامَ عِنْدَهَا سَبْعًا وَإِذَا تَزَوَّجَ الثَّيِّبَ عَلَى الْبِكْرِ أَقَامَ عِنْدَهَا ثَلاَثًا. قَالَ خَالِدٌ وَلَوْ قُلْتُ إِنَّهُ رَفَعَهُ لَصَدَقْتُ وَلَكِنَّهُ قَالَ السُّنَّةُ كَذَلِكَ.

It was narrated that Anas bin Malik said: "If (a man) marries a virgin in addition to a previously-married woman, he should stay with her for seven days, and if he marries a previously-married woman in addition to a virgin, he should stay with her for three." Khalid (a narrator) said: "If I were to say that he attributed it to the Prophet (s.a.w) I would be speaking the truth, but he said: 'That is the Sunnah.'"