It is narrated from Mujahid (may Allah have mercy on him) that a Jew passed by some people who were in a mosque while they were saying "Amin" (at the end of a supplication). The Jew said (on seeing what they did): "By the One who taught you 'Amin' (O Allah, accept our supplication), you are certainly upon the truth."
The chain of this report is jayyid (good), and Imam Musaddad recorded it in his Musnad.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / صفة الصلاة / 475
Hadith Gradingمحدثین:إسناده ضعيف
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :إسناده ضعيف. » «انفرد به المصنف من هذا الطريق»