When to Stand for Prayer during the Iqamah
Question

When the muezzin starts to call some brothers stand up and starts to arrange the rows and some brothers still wait where they are till the muezzin calls “qad qamat as-salah” , So What is the authentic way?
Praise be to Allah.The matter is broad in scope and a person may stand up at the beginning of the Iqamah or in the middle of it or after it finishes. The majority of scholars are of the view that if the Imam is in the mosque, they should not stand up until the Iqamah ends.
Imam Malik (may Allah have mercy on him) said in Al-Muwatta’:
With regard to people standing up when the Iqamah for prayer is given, I have not heard anything definitive concerning that, but I think it depends on what the people are able to do, because there are heavy and light ones among them and they are not all able to stand up at the same time. (End quote)
Al-Hafidh Ibn Hajar (may Allah have mercy on him) said in Al-Fat-h:
The majority are of the view that if the Imam is with them in the mosque, they should not stand up until the Iqamah ends.
It was narrated that Anas used to stand up when the Mu’adhdhin said “Qad qamat as-salah (prayer is about to begin)”(Narrated by Ibn Al-Mundhir and others)
It was also narrated by Sa`id ibn Mansur via Abu Is-haq that the companions of `Abdullah (i.e., Ibn Mas`ud, may Allah be pleased with him) also did that. It was narrated that Sa`id ibn Al-Musayyib said: When the Mu’adhdhin says “Allahu akbar” one must stand up. When he says “Hayya `ala As-Salah (come to prayer), the rows should be straightened, and when he says “La ilaha illallah” the Imam should say Takbir (to start the prayer). But if the Imam is not in the mosque, the majority are of the view that they should not stand up until they see him. (End quote)
Shaykh Ibn `Uthaymin (may Allah have mercy on him) was asked: Is there any mention in the Sunnah of a specific point in the Iqamah when people should stand up for the prayer?
He replied:
There is no mention in the Sunnah of a specific point in the Iqamah when people should stand up for the prayer, but the Prophet (peace and blessings of Allah be upon him) said: “Do not stand up until you see me.” (Narrated by Al-Bukhari and Muslim)
If a person stands up at the beginning of the Iqamah or during it or when it ends, all of that is permissible. (End quote from Majmu` Fatawa Ash-Shaykh Ibn `Uthaymin, 13/16)
And Allah knows best.