Muhammad bin Ubaydullah al-Thaqafi narrates that Sayyiduna Amr bin Hurayth (may Allah be pleased with him) used to accept his (the hidden witness's) testimony, and he would say: "This is how one deals with the treacherous and the wicked."
Imam al-Shafi'i (may Allah have mercy on him) said that Sayyiduna Umar (may Allah be pleased with him) accepted the testimony of those who watched a man committing zina, but they did not make up four.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21190