Ref. No. 41/1065
In the name of Allah the most gracious the most merciful
The answer to your question is as follows:
In Fiqh books, standing up at the time of ‘Hayya-alas-salah’ is regarded as Mustahab. Allama Tahtavi writes that standing up at the time of ‘Hayya-alas-salah’ means one should compulsorily stand up at ‘Hayya Alas Salah’ in any way and he is not allowed to keep sitting after the ‘hayya alas salah’ is called. So, it never means that one is not allowed to stand up before ‘hayya alas salah’.
Hence, imam’s insistence over standing up only at the time of Hayya alas salah and not before is absolutely wrong. You should keep in mind that aligning rows (saff) is wajib (mandatory). And acting upon a wajib is more important than a mustahab. The companions of the prophet (Muhammad) (saws) used to stand up the time when they saw the prophet (saws) coming in masjid. At the same time, hazrat bilal would start takbeer. It denotes that standing up at the beginning of takbeer is also approved. Nevertheless this looks good as regards aligning rows so that the aligning of the rows can be gained before the takbeer gets over.
عن ابی ھریرۃ رضی اللہ عنہ یقول اقیمت الصلوۃ قمنا فعدلنا الصفوف قبل ان یخرج الینا رسول اللہ (مسلم1/220) اشارۃ انہ ھذہ سنۃ معھودۃ عندھم (نووی شرح مسلم 1/221)
It is narrated by hazrat Abu hurairah (ra) that takbeer started, we stood up and aligned rows before the prophet (saws) came out to us. While elaborating this narration, Allama Nawavi writes: it indicates that this (standing up at the beginning of takbeer) is the sunnah in their view. (Nawavi Sharhe-Muslim 1/220). Hazrat Hafiz ibne Hajar writes in his book namely ‘Fat’hul bari Sharhe Bukhari’ narrating from ibne shihab which openly states that they used to stand up in rows at the time takbeer started.
روی عبدالرزاق عن ابن جریج عن ابن شھاب أَنَّ النَّاسَ كَانُوا سَاعَةَ يَقُولُ الْمُؤَذِّنُ اللَّهُ أَكْبَرُ يَقُومُونَ إِلَى الصَّلَاةِ فَلَا يَأْتِي النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَقَامَهُ حَتَّى تَعْتَدِلَ الصُّفُوفُ۔ (فتح الباری 2/95)
And Allah knows best
Darul Uloom Waqf Deoband