Prerequisites of Salaah are six:
1) Entry of time:
Starts: From the time Sun starts moving after it had reached and settled in its zenith.
Ends: When the length of an object’s shadow becomes equal to the length of the object itself.
Bare in mind that when the Sun is at its zenith and above the object, it’s shadow does not completely disappear (except at the equator); this small shadow is called Fay al-Zawal (فيء الزوال). This is important because whilst measuring the shadow of an object to see if it is the same length of the object one is required to add on the length of the Fay al-Zawal. For example, the length of the object is 10 metres, and when the Sun was above this object the Fay al-Zawal was 50 cm, so whilst measuring the length of the shadow the time of Dhuhr would have come to an end with the length is 10.5 metres.
Starts: From the end of the time of Dhuhr.
Ends: When the sun has set.
Note: ‘Asr has two times:
a) Waqtu-l Ikhtiyar – This is from the end of the time of Dhuhr until the shadow of every object has reached twice its length. This is the ‘time of preference’ and ‘Asr shouldn’t be delayed beyond this time without a valid excuse, otherwise one is sinful.
b) Waqtu-d Darurah – From the time the shadow of every object is twice its length until the sun has set. This is the ‘time of necessity’ and ‘Asr should not be prayed during this time without valid excuse as stated previously. However, if one still prays at this time without a valid excuse, their Salah is valid but they are sinful.
Starts: After the sun has set.
Ends: When the redness of the sunset disappears from the horizon.
Note: Maghrib also has two times according to our Madhhab, as stated in al-Insaf and al-Iqna’:
a) Waqtu-l Istihbab – Between sunset until the appearance of the stars. This is the preferred time.
b) Waqtu-l Karahah – If one delays Maghrib until the stars have started to appear in the sky then this is disliked. One is not sinful for doing so but avoiding it is better.
Starts: After the disappearance of the redness of sunset.
Ends: When the time for Fajr enters, which is the second dawn (al-Fajr al-Sadiq): when there is whiteness along the horizon, as opposed to whiteness vertically spiking up from the horizon like pillars (al-Fajr al-Kadhib/the false dawn).
Note: ‘Isha has two times:
a) Waqtu-l Ikhtiyar – Between the disappearance of the redness of sunset until the first third of the night (which is calculated by splitting the time between Maghrib and Fajr into three).
b) Waqtu-d Darurah – After the first third of the night has elapsed until Fajr. If one delays ‘Isha until the first third is over without a valid reason then they are sinful even though the Salah is accepted.
Starts: From second/true dawn (explained above).
Ends: When the sun begins to rise.
The rest of the conditions (Purity from Hadath; Concealment of the ‘Awrah; Avoidance of impurities; Facing the Qiblah; Intention) will be covered in separate posts, InshaAllah
This answer was collected from The Hanbali Madhhab website. The answers to the questions are given by Imam Zahed Fattah in accordance to the Hanbali School of Jurisprudence.