The Prophet (sallal-laahu-alayhi-wasallam) used to command Bilaal to leave an interval between his Adhaan and Iqaamah so that two units of (prayer can be prayed). There were those amongst the companions who prayed two units of prayer between the Adhaan and the Iqaamah and the Prophet (sallal-laahu-alayhi-wasallam) saw them and approved their (action). He (sallal-laahu-alayhi-wasallam) said: ”There is a prayer between the two calls (i.e. Adhaan and Iqaama), there is a prayer between the two calls.” And whilst saying it the third time he said: “For the one who wishes”, fearing that it would be taken as something binding in his Sunnah. So if the Mu’addhin leaves an interval for that (two units of prayer), then this prayer is something good; but if he calls the Iqaamah straight away after calling the Adhaan, then busying oneself in responding to the Mu-addhin is the Sunnah. That is because the Prophet (sallal-laahu-alayhi-wasallam) said: ”If one of you hears the Mu-addhin, then let him say what he says” (i.e. he repeats what the Mu-addhin says).
And it is not befitting for anyone to leave responding to the Mu-addhin and prays those two units of prayer, because the Sunnah is that one who hears the Mu-addhin should say what the Mu-addhin is saying. Then he should send the salutation upon the Prophet (sallal-laahu-alayhi-wasallam) and says: ‘………….’اللهم رب هذه الدعوة التامة O Allaah! Lord of this perfect call…..” to the end of the supplication… [Source:Al-Fataawa al-Kubraa of Shaikhul Islaam Ibn Taymiyyah (rahimahullaah) Vol: 2: page: 270-271]