Sahih Bukhari | Volume 7 | Hadith Number 141

Narrated by Anas
It is the Prophet's tradition that if someone marries a virgin and he has already a matron wife then he should stay for seven days with her (the virgin) and then by turns; and if someone marries a matron and he has already a virgin wife then he should stay with her (the matron) for three days, and then by turns.