Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Prophet (peace and blessings be upon him) was on the pulpit when suddenly he said three times: "Amin." He (peace and blessings be upon him) was asked about that, and he (peace and blessings be upon him) said: "Jibril (peace be upon him) came to me and said: Whoever has you mentioned in his presence and does not send blessings upon you, may Allah distance him; say: Amin. So I said: Amin. Whoever lives to see his parents, or one of them, and then dies without being forgiven, may Allah distance him; say: Amin. So I said: Amin. And whoever lives to see Ramadan and is not forgiven, may Allah distance him; say: Amin. So I said: Amin."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11069