Answered by Shaykh Faraz Rabbani
If I start my Dhuhr prayer in Dhuhr time, but by the time I’ve finished my Dhuhr prayer, asr time has started. Is my Dhuhr prayer valid? Is it the same ruling for the other prayers? I’ve heard there is a special ruling regarding the asr prayer, can you please elaborate on this issue?
It is sinful to delay the prayer such that some of it falls outside its time. However, this does not invalidate the prayer nor make it a makeup prayer, except for fajr.
When the sun rises [i.e. any part of the orb of the sun appears on the horizon], the fajr prayer is invalidated.
Response from Shaykh Abu Usamah
The Zuhr prayer is valid. If the sun rises whilst offering Fajr the salah becomes invalid. Asr salah is valid if the sun sets whilst still engaged in it.