Sunan Abi Dawud, Hadith 2119

Narrated Anas b. Malik : When a man who has a wife married a virgin he should stay with her seven nights ; if he marries to a woman who has been previously married he should stay with her three nights. (The narrator said:) If I say that he (Anas) narrated this tradition from the Prophet I shall be true. But he said: The Sunnah is so-and-so.

حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا هُشَيْمٌ، وَإِسْمَاعِيلُ ابْنُ عُلَيَّةَ، عَنْ خَالِدٍ الْحَذَّاءِ، عَنْ أَبِي قِلاَبَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ إِذَا تَزَوَّجَ الْبِكْرَ عَلَى الثَّيِّبِ أَقَامَ عِنْدَهَا سَبْعًا ‏.‏ وَإِذَا تَزَوَّجَ الثَّيِّبَ أَقَامَ عِنْدَهَا ثَلاَثًا ‏.‏ وَلَوْ قُلْتُ إِنَّهُ رَفَعَهُ لَصَدَقْتُ وَلَكِنَّهُ قَالَ السُّنَّةُ كَذَلِكَ ‏.‏

sunnah.com

also referenced under: Hadith 2124 (DarusSalam)