It is narrated from Ata (may Allah have mercy on him) that he said: "In matters concerning women, the testimony of women, when there is no man with them, is not permissible with less than four just women."
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1707