ḤADĪTH OF THE DAY

“ Al- Ḥakam Ibnul-A'raj said, "I came to Ibn-'Abbās , who he was bolstered on his cloak at Zamzam, and said, 'Tell me which day is the Day of 'Āshūrā' (2) so that I fast it.' He replied, 'When you see the crescent (3) of [the month of] Muharram, count [the days] then begin fasting from the ninth day (4).' I asked, 'Is this how Muhammad, peace and blessings of God be upon him, used to fast ['Āshūrā']?' He confirmed, 'yes.'" Footnotes: 1. The well located in the vicinity of the Sanctified House of God, the Ka'bah, where the prophet Ishmael's mother, Hagar, found water under her son's feet. The well produces water to this day. 2. The day on which prophet Moses used to fast to give thanks to God for saving the Children of Israel from their enemy. Fasting on the Day of 'Ashūrā' is optional. However, the Prophet, peace and blessings of God be upon him, said that the reward for fasting on that day is the expiation of sins committed during the year leading up to it. 3. The appearance of the crescent signals the beginning of the month. 4. The majority of scholars consider the Day of 'Āshūrā' to be the tenth of Muharram. Ibn-'Abbās also narrated another hadīth clearly identifying the tenth of Muharram as the Day of Āshūrā'. However, the Jews of Madīnah used to fast on that day long before the arrival of the Prophet, peace and blessings of God be upon him. In order to distinguish the Muslims from them, the Prophet, peace and blessings of God be upon him, decided to fast an extra day the following year; only he did not live long enough to do so. In which case, he would indeed have started to fast the morning of the ninth of Muharram, as indicated by Ibn-'Abbās. ”

Sunan at-Tirmithiy كتاب الصلاة باب ما يقول عند افتتاح الصلاة

حَدَّثَنَا قُتَيْبَةُ حَدَّثَنَا اللَّيْثُ عَنِ ابْنِ شِهَابٍ عَنْ أَبِى سَلَمَةَ عَنْ أَبِى هُرَيْرَةَ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِنَّ الشَّيْطَانَ يَأْتِى أَحَدَكُمْ فِى صَلاَتِهِ فَيَلْبِسُ عَلَيْهِ حَتَّى لاَ يَدْرِى كَمْ صَلَّى فَإِذَا وَجَدَ ذَلِكَ أَحَدُكُمْ فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ. قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ.
The Messenger of God, peace and blessings of God be upon him, said, "Indeed, Satan comes to anyone of you while in prayer, and causes you confusion until you do not know how much you have prayed [1]. So, if anyone of you finds this, then you should prostrate twice from the seated position."
[1] i.e. how many units of prayer.