It is narrated from Ibrahim (may Allah have mercy on him) that, concerning the saying of Allah Almighty: "And it is not lawful for them to conceal what Allah has created in their wombs" [Surah al-Baqarah: 228], he said: Most of what is meant by it is menstruation.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15413