What constitutes having 'caught' the prayer time for Dhuhr or Asr?
Chapter on Prayer Times
Al-Mughni
Book of Prayer
Primary text
Catching the time for a prayer, whether due to an excuse or not, is established by completing at least one Rak'ah before the time expires. If one completes a single Rak'ah of Asr before sunset, they are considered to have performed Asr within its time. This applies to all prayers; whoever catches one Rak'ah of any prayer within its time is considered to have caught the prayer, based on the agreed-upon Hadith: 'Whoever catches one Rak'ah of the prayer has caught the prayer,' and specifically for Asr, 'Whoever catches one Rak'ah of Asr before the sun sets has caught the Asr prayer.' There is no known disagreement on this principle.
Supporting text
The permissibility of delaying the prayer past the preferred time is restricted to necessity and valid excuses, such as a menstruating woman becoming pure, a disbeliever embracing Islam, a child reaching maturity, an insane person regaining sanity, a sleeper waking up, or a sick person recovering.