Prophet (saw) said Ameen thrice when Jibreel come and said..
Kaab Ibn Ujrah (RA) relates that Rasulullah (SAW) said Come near to the mimbar and we came near the mimbar. When He (SAW) climbed the first step of the mimbar, He (SAW) said "Aameen", When He (SAW) ascended the second step, He (SAW) said "Aameen", When He (SAW) climbed the third step, He (SAW) said "Aameen" When He (SAW) came down, We said "O Rasool of Allah (SAW), we have heard from you today something which we never heard before" He (SAW) said When I climbed the first step, the angel Jibraeel (A.S.) appeared before me and said
"Destruction to him who found the blessed month of Ramdhan and let it pass by without gaining forgiveness"
upon that I said 'Aameen'. When I climbed the second step, he said,
"Destruction to him before whom thy name is taken and then he does not make Dua for Allah's blessing on me (by saying, for example SAW)."
I replied 'Aameen'. When I climbed the third step, he said
"Destruction unto him in whose lifetime his parents or either one of them reaches old age, and (through failure to serve them) he is not allowed to enter Jannah".
I said 'Aameen'. (Hakim, Baihaqi)
Note In this Hadith, it appears that Jibraeel (AS) gave expression to three curses, upon which Rasulullah (SAW) said Aameen every time. In al-Durr al-Mansoor it is reported that Jibraeel (AS) advised Rasulullah (SAW) to say Aameen. Being an angel of such high mark, Jibraeel 's giving these curses is sure to be accepted. May Allah (Subhanahu wa Taala) in His infinite mercy grant us His help and save us from these three dangers.