Questioner: If someone is making Du’aa, should we say respond by saying ‘In shaa Allaah’ or ‘Aameen’?
Dr Sharof: Firstly, if someone makes Du’aa for you, you should say Aameen. As for ‘In shaa Allaah’, what is the evidence for it? What is the evidence that mentioned that you should say ‘In shaa Allaah’ when a person makes Du’aa for you? There is none. (So) you should say Aameen. The response to Du’aa is Aameen. That is why at the end of the recitation of Sooratul Faatihah in Solaat the congregation will say Aameen. In fact, al-Imaam As-suyootee (rohimohuLlaah) mentioned it in his book, ‘Al-Itqaan fee ‘Uloom al-Qur’aan’. He said when Sooratul Faatihah was revealed, angel Jibreel ordered the Prophet to say Aameen after he recited it, even when he was not praying at the time.
And we have also seen that Prophet Moosa was supplicating while Prophet Haaroon was saying Aameen. Allaah responded by saying ‘You have been granted your request…’ [Soorah Taa Haa (20):36] The one who is praying and the one who is saying Aameen are both supplicating. This is because the one who says Aameen is also supplicating. The meaning of Aameen is ‘Allaahumma stajib (O Allaah accept the supplication)’. For this reason, if someone prays for you, you should say Aameen. If anyone says you should not say Aameen, then he is the one trying to take you away from knowledge. If you say the response is ‘In shaa Allaah’, produce your proofs if you are indeed truthful.
SOURCE: [Question and Answer session of the tape titled, “Explanation of the names and attributes of Allaah”]