It is narrated from Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When seven (days) have passed for a woman in nifas and she then sees purity, she should perform ghusl and pray."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1617