It is narrated from Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him): If a menstruating woman becomes pure before the sun sets, she will pray Zuhr and Asr together, and if she becomes pure before dawn, she will pray Maghrib and Isha together.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1815