What is the ruling on a believer saying "Allahumma Istajib" (O Allah, Grant) after finishing a supplication, or upon hearing the supplication of others? And does this contradict the Sunnah? And is it better to say "Amin" (Amen) or are both phrases the same?

After supplicating, it is permissible for a Muslim to say: "O Allah, answer my prayer," or "O Allah, answer his prayer," or "Ameen." All of these convey the meaning of requesting a response, and they are part of the' prescribed supplication. An exception to this is saying "Ameen" upon completing Al-Fatiha in prayer, as this is what the Sunnah indicates.