It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, among the former nations, a hundred (people) constituted an ummah (a body of witnesses); when they testified to the good of a servant, Paradise became obligatory for him. And in my Ummah, fifty of them constitute an ummah; so when they testify to the good of a servant, Paradise becomes obligatory for him."
Imam Abu Ya'la narrated this hadith in his Musnad (4183). Its chain contains the narrators Zaynab bint Yazid and Harb bin Surayj, on account of whom some imams have criticised its chain; however, its meaning is established by other authentic hadiths (such as "You are the witnesses of Allah on earth").
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب المناقب / 4183
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :الحديث بهذا الإسناد فيه حرب بن سريج وهو صدوق يخطئ. » «أخرجه مسند أبي يعلى برقم: 4369»