Abu al-Musabbih al-Maqra'i (may Allah have mercy on him) said: We used to sit with Sayyiduna Abu Zuhayr al-Numayri (may Allah be pleased with him), and he was one of the Companions of the Prophet (peace and blessings be upon him). When he spoke, he spoke excellently. When any man among us made a supplication, he would say: Seal it with Amin, for Amin in a supplication is like the seal on a document. Sayyiduna Abu Zuhayr (may Allah be pleased with him) said: I will tell you about that: One night we went out walking with the Messenger of Allah (peace and blessings be upon him), and we came upon a man in a tent who was supplicating with great insistence. The Messenger of Allah (peace and blessings be upon him) stopped to listen to his supplication, then the Messenger of Allah (peace and blessings be upon him) said: "If he seals it, he has made it binding (that it be answered)." A man among the people said to him (peace and blessings be upon him): O Messenger of Allah, with what should he seal it? He (peace and blessings be upon him) said: "With Amin; if he seals it with Amin, he has made it binding." Then the man who had asked the Messenger of Allah (peace and blessings be upon him) went (to the one supplicating) and said: O so-and-so, seal it with Amin and rejoice at the good news.
Hadith Referenceالمعجم الكبير للطبراني / مسند من يعرف بالكنى / 18960