No prayer is valid after the 'Asr prayer till the sun sets and no prayer is valid after the dawn prayer till the sun rises.
The Prohibition of Prayer at Specific Times
This narration from Sahih Muslim 827 establishes two distinct periods during which voluntary prayers are forbidden according to Islamic law.
First Prohibited Period
After the 'Asr prayer until sunset - This period marks the final portion of the day when the sun begins its descent. The wisdom behind this prohibition includes avoiding resemblance to sun-worshippers and maintaining the distinct timings prescribed for Islamic prayers.
Second Prohibited Period
After the Fajr (dawn) prayer until sunrise - This early morning period is protected from voluntary prayers to preserve the emphasis on the obligatory Fajr prayer and to prevent confusion between prescribed and supererogatory worship.
Scholarly Exceptions
The majority of scholars hold that this prohibition applies only to voluntary prayers, while missed obligatory prayers may be made up during these times. Some schools also permit prayers with specific causes, such as greeting the mosque or making up sunnah prayers associated with prior obligatory prayers.
Divine Wisdom
These restrictions teach Muslims to respect divinely appointed timings and cultivate spiritual discipline. They prevent the dilution of prayer's significance through constant repetition and help maintain the unique spiritual atmosphere of each prayer time.