What Is the "Best Time" for Prayer, and Is It Required?
Quick answer: The "best time" is simply the earliest part of a prayer's window — recommended, but not required, since the prayer remains valid throughout its full time.
Scholars divide each prayer's window into a "best time" (fadilah) — the earliest portion, recommended for the greatest reward — and a "valid time" (jawaz), the remainder of the window during which the prayer is still fully valid.
The virtue of praying early is supported by several hadiths, including the Companions asking the Prophet ﷺ about the best deeds, with "prayer at its (earliest) time" mentioned first — indicating it's recommended, not obligatory.
As an exception, it's recommended to delay Isha slightly past its earliest time (though not to its end) for those who find that manageable, and to delay Dhuhr slightly in intense heat if worshippers have to walk a tiring distance to reach the mosque.
In short: there's no sin in praying at any point within a prayer's designated window, but praying as early as possible carries greater reward according to most scholars, aside from the noted exceptions.