Riyad us Saliheen: Hadith 1142

I saw some people performing Duha (prayers) in the early forenoon and warned them (saying): These people must know that performing Salat a little later is better. The Messenger of Allah (ﷺ) said, "The Salat of the penitent is to be observed when the young ones of camels feel the heat of the sun (i.e., when it becomes very hot)."

[Muslim].