Ibn Sirin (may Allah have mercy on him) said: When a man buys a young slave girl who has not yet reached the age of menstruation, he observes istibra' (waiting to establish that the womb is free) with her for three months.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب عتق أمهات الأولاد / 21811