Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.103A.

Section : When One Should Stand for the Prayer.

Malik states  in al-Muwatta, “I have not heard anything concerning the specific time to stand for prayer. I have seen some people lagging and others being quick.” Ibn al-Munzhir recorded that Anas would stand when Qad qaamtus-salah was said.