It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) ascended the pulpit and said "Amin" three times, then said: "Do you know why I said Amin?" The Companions said: Allah and His Messenger know best. He (peace and blessings be upon him) said: "Jibril (peace be upon him) came to me and told me that whoever has me mentioned in his presence and does not send blessings upon me shall enter the Fire; may Allah remove him far (from His mercy) and humiliate him. So I said: Amin. And whoever finds his parents, or one of them, and does not treat them with kindness shall enter the Fire; may Allah remove him far and humiliate him. So I said: Amin. And whoever reaches Ramadan and is not forgiven shall enter the Fire; may Allah remove him far and humiliate him. So I said: Amin."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12498