Entrance of the prayer time
For every compulsory prayer , there is a specific time before and after which the Salat would be invalid, except for necessities.
Allah (glorified be He) says: “Verily, as-Salat is enjoined upon the believers at fixed times.” (An-Nisa’: 103).
This means that it is compulsory at scheduled times.
It was reported on the authority of Abu Musa Al-Ash’ari , “A questioner came to the Messenger of Allah ﷺ asking him about the Salah and he did not respond. Following that he established the Fajr Salat at its very beginning; being so dark that people could not recognize one another. When Dhur time came he commanded and established it at the time when the sun was descending from its mid-point; in a manner that the onlooker would have said that it was the middle of the morning (but he was more knowledgeable than them). At ‘Asr time he commanded and performed ‘Asr and the sun was still raised in the sky. When Maghrib time came he did the same as the sun had fallen (below the horizon). ‘Isha was established at the time when the twilight had disappeared. Following this on the next day he delayed Fajr until the onlooker would say after leaving from the prayer: “The sun has risen! Or has almost (risen)!” He delayed Dhur until it became close to the time of ‘Asr prayer of the previous day and he delayed ‘Asr until when the one who had finished praying were to look and say that the sun has become red. He delayed Maghrib until it was at the time when the twilight dissipates. Finally he delayed ‘Isha until the first third of the night had passed. Following that the next morning he called the questioner and told him that the time was between those two.” In another narration “Then he prayed Maghrib before the twilight dissipated on the next day.” [ Source: Muslim.]
The prayer times are:
1- Time for Fajr (dawn) prayer:
From the appearance of the true dawn - the whiteness (white light) that appears from the direction of the east - until when the sun rises.
2- Time for Dhur (noon) prayer:
From when the sun moves away from the meridian (post meridiem) until when the shadow of an object is up to its length, (i.e. this shadow is) after the first shadow cast by the sun before it changes direction away from the meridian. This is because every object casts its shadow towards the west and this diminishes continuously as the sun rises. Eventually the shadow seizes to diminish and thereafter starts to increase in length (elongate). The time when this elongation starts marks the midday (meridiem).
3- Time for ‘Asr prayer:
From the end of the time of Dhur prayer until when the shadow of an object doubles its height (length), (i.e. this shadow is) after the first shadow cast by the sun before it changes direction away from the meridian.
4- Time for Maghrib (sunset) prayer:
From the sunset until when the red glow disappears. The red glow is the red light which appears at the horizon during the course of the sun setting.
5- Time for ‘Isha prayer:
From the end of Maghrib’s time until the middle of the night, since the Prophet ﷺ said: “…the time of Salat ul-‘Isha extends to the middle of the night.” [ Source: Muslim.]
However, in this day and age the prayer times can be known easily through the use of prayer calendars.