Praise be to Allah.
The scholars (may Allah have mercy on them) differed concerning the point at which one should stand up to pray. There are several opinions that were mentioned by an-Nawawi in al-Majmoo‘ (3/233), and they are as follows:
(i)That he should stand up when the muezzin starts to recite the iqaamah. This is the view of ‘Ata’ and az-Zuhri
(ii)That he should stand up when the muezzin says “hayya ‘ala as-salaah (come to prayer)”. This is the view of Abu Haneefah.
(iii)That he should stand up when the muezzin has completed the iqaamah. This is the view of ash-Shaafa‘i.
(iv)That there is no specific point at which one should stand up; rather it is permissible for the worshipper to stand up at the beginning of the iqaamah, during it or at the end of it. This is the view of the Maalikis.
(v)That it is Sunnah to stand up when the muezzin says “Qad qaamat is-salaah (prayer is about to begin)” if the worshipper can see the imam; if he cannot see him, then he should stand up when he does see the imam. This is the view of Imam Ahmad.
There is no clear evidence from the Sunnah to support any of these opinions; rather this is the ijtihaad worked out by the imams according to what each of them thought was correct.
Based on that, the matter is broad in scope, so the worshipper may stand up whenever he wants, at the beginning of the iqaamah or during it. But the Sunnah indicates that if the muezzin gives the iqaamah for prayer when the imam has not yet entered the mosque, then the worshippers may not stand up until they see him.
It was narrated that Abu Qataadah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “When the iqaamah for prayer is given, do not stand up until you see me.”. Narrated by al-Bukhaari (637) and Muslim (604). According to a version narrated by Muslim: “Until you see that I have come out.”