It is narrated from Hassan bin Atiyyah (may Allah have mercy on him) that he said: "If a monk (a secluded Christian worshipper) supplicates for you, there is no harm in saying Amin to his supplication." He also said: "Indeed, their supplications in our favour (the people of faith) attain acceptance in the court of Allah, but their supplications for their own selves are not accepted."
Imam Ishaq bin Rahwayh narrated this athar (report) in his Musnad, and its chain is sahih (authentic).
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الأذكار والدعوات / 3370
Hadith Gradingمحدثین:إسناده صحيح، مقطوع
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :هذا إسناد مقطوع، رجاله كلهم ثقات. » «أخرجه مصنف ابن أبي شيبة برقم: 30454»