Kulthum bin al-Aqmar (may Allah have mercy on him) narrates from Judge Shurayh (may Allah have mercy on him) that the testimony of one who hid himself (to witness) is not acceptable.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21188
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