Sahih Muslim Book 8, Hadith Number 3448.
Chapter: How long after marrying one should stay with a virgin or a woman previously married.
Anas b. Malik reported: When anyone who has already a wife marries virgin, he should stay with her for seven nights (and then turn to his other wife), but when anyone having a virgin with him (as his wife) marries a woman who has been previously married he should stay with her for three nights. Khalid (one of the narrators) said. If I were to say that it could be directly traced to the Holy Prophet (May peace be upon him). I would have told the truth, but he (Hadrat Anas) said: Such is the tradition.