Abu al-Malih bin Usamah al-Hudhali narrates: My father, Sayyiduna Usamah (may Allah be pleased with him), told me from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "When forty or more people from any community bear witness, Allah accepts their testimony" - or he said: "He confirms their testimony."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 499