Al-Baqara (The Cow)
Chapter 2: Verse 186
"And when My servant ask you concerning Me, then verily I am near, I answer the prayer of the supplicant when he calls on Me, so they should answer My call and believe in Me, so that they may walk in the right way."
The whole verse is based on the first person singular pronoun which shows the great importance which the speaker, i.e. God, attaches to this subject. Then come the words My servants and not "people" or other such words. This enhances its importance even more. The reply (then verily I am near) starts dramatically just after the question without any preamble like, "then say that I am near." The reply is emphasized with "verily." The nearness of the speaker is described with the adjective near and not with any verb like "I come near him." Thus it shows that He is already near and will always remain near. Then the answer (I answer the prayer of the supplicant) is conditional upon when he calls on Me.
These points show how much importance has been attached to the answering of the prayers. In addition, this verse - short as it is - repeats the first person singular pronoun seven times; it is the only verse with this characteristic in the whole of Quran.
In short, God's ownership of His creatures gives Him authority to answer any prayer which is put before Him by any of His creatures, and to fulfill his or her need by accepting his or her supplication.
This verse not only mentions a fact, i.e. answering prayer, but gives its reasons also: Because the supplicants are servants of God, He is near to them; and because He is near to them, He answers their prayers without any reservation. Whatever prayer is addressed to Him, He shall answer it.
The condition when he calls on Me shows that the promise of answering the prayer shall apply when the supplicant is a supplicant in reality; when he or she wants that thing according to his or her natural and deep-ingrained knowledge and when his or her heart is really in what his or her tongue is asking for.
Therefore, the natural prayer addressed to God shall always be answered. If a prayer is not answered, then it lacks both or one of the two things mentioned in the verse: the prayer of the supplicant when he calls on Me. It may happen in the following ways:
First: There may be no prayer at all. For example, a person prays for an impossible thing (but he or she does not know that it is impossible), or for a thing which, if he or she knew the fact, would not have wanted at all.
Second: There is indeed a prayer, but it is not addressed exclusively to God.
God is near to His servants. Nothing comes between Him and their prayer. He cares for them and for the things they ask for. That is why He invites them to call upon Him and He is of such high attributes. Therefore, they should accept this invitation of their Lord and should advance towards Him, and have faith in Him about this attribute, having firm belief that He is near and He answers their call; so that they may be guided rightly in praying unto Him.
Ramadan: Motivating Believers To Action, "Quranic Commentary on 'I Answer The Prayer'" - Muhammad Husayn Tabataba'i , pp. 94-99