It is narrated from Ikrimah bin Khalid al-Makhzumi that a woman was brought to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) who had testified against a man and his wife that she had nursed them both. He (may Allah be pleased with him) said: No, not until two men, or one man and two women, testify. Abu Sa'id bin Abi Amr informed us: al-Rabi' informed us; he said: Imam al-Shafi'i (may Allah have mercy on him) informed us: Muslim informed us from Ibn Jurayj, who narrates from Ata' that he said: The testimony of fewer than four women is not acceptable.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرضاع / 15676