salamun alaykum.may Allah bless you.dear Sheikh I read in hadith that during battle of khandaq the prophet peace be upon him did not have opportunity to pray asr in its due time till nearly sunset then he performed ablution and prayed it after sunset.how can I reconcile this with scholars statement that if you fear time will be over you have to perform tayammum.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Scholars held different opinions regarding the case when a person fears that the prescribed time of the prayer would expire if he performed Wudhoo' (ablution) or Ghusl (ritual bath) using water, whether he should perform it anyway and then pray even after the end of the prayer's time, or perform Tayammum (dry ablution) and pray so as not to miss the prayer. Scholars held two different opinions in this regard. Most of scholars maintained that when water is available, but the person fears that the prayer’s time will expire if he performs Wudhoo' or Ghusl using water, he should perform Wudhoo' or Ghusl normally and then perform the prayer even if after its time has expired. However, the Maaliki scholars held that one should perform Tayammum and pray before the prayer’s time expires. The Hanafi scholar Zufar held the same opinion.

Here are some statements of scholars on this issue:

The Hanbali scholar Ibn Qudaamah said, “If water is available, but bringing it and performing Wudhoo' or Ghusl with it means that the prayer’s time will expire, it is impermissible for the Muslim to perform Tayammum in this case whether he is a resident or travelling, according to most of the scholars, including Ash-Shaafi‘i, Abu Thawr, Ibn Al-Munthir, Abu Haneefah and his followers. It is reported from Al-Awzaa‘i, and Ath-Thawri that it is permissible for such a person to perform Tayammum.” [Al-Mughni]

It is stated in Mughni Al-Muhtaaj, a book on the Shaafi`i school of Fiqh, that, “On the contrary, when water is available, but the person fears missing the prayer in its due time if he performs ablution, he has to perform ablution. He is not allowed to perform Tayammum in this case; because water is available…”

The Hanafi scholar Ibn Nujaym said, “It is invalid to perform Tayammum for fear of missing Friday prayer or any obligatory prayer in its due time. Performing Tayammum is allowable only when water is not available in actuality or virtually (as entailed by a Sharee'ah ruling). This is contrary to the opinion of Zufar as underlined earlier. As for ruling that it is impermissible to perform Tayammum for fear of missing Friday prayer, this is because the person still has an alternative by performing Thuhr prayer if he has missed the Friday prayer... Similarly, it is impermissible to perform Tayammum for fear of missing any other obligatory prayer because the person has an alternative by making up the missed prayer.” [Al-Bahr Ar-Raa’iq]

As for the Hadeeth to which the questioner referred in the question about delaying ‘Asr prayer during the Battle of the Trench until after sunset, it was not because of the unavailability of water. Instead, he, sallallaahu ‘alayhi wa sallam, delayed the prayer because of his preoccupation with fighting the enemy. Other scholars suggested that it was out of forgetfulness. The Haafith Ibn Hajar wrote, “Scholars held different opinions regarding the reason behind delaying ‘Asr prayer on the day of the Trench; some maintained that the Prophet, sallallaahu ‘alayhi wa sallam, forgot to perform it; however, it seems unlikely that all the Companions forgot as well. Some other scholars held that the Prophet, sallallaahu ‘alayhi wa sallam, delayed it deliberately because he was so busy fighting the disbelievers that he was not able to offer the prayer on its due time. This opinion is likely to be the correct one, especially since Abu Sa‘eed Al-Khudri reported that this incident took place before the 'Fear Prayer' was prescribed as reported by Ahmad and An-Nasaa’i ...” [Fat-h Al-Baari]

Allaah Knows best.