“I once asked abu Ja‘far (a.s.), about how much of Salah (prayer) Allah, the most Majestic and Glorious, has made obligatory. He (the Imam) said, ‘During one day and night five prayers are made obligatory.’ I then asked, ‘Has He named and explained them in His book?’ He replied, ‘Yes, Allah, the most High, has said to His Holy Prophet (sw), “Perform prayer after the sun declines to the west, on midday until the darkness of the night.” (17: 78). The sun declines on midday. There are four prayers between midday and the darkness of the night. Allah has named, explained and timed them. Darkness of night is in midnight. He, most Blessed, most High has then said, “...and the recitation (performing prayer) at dawn and recitation at dawn was witnessed (attended by the angels of the day and those of the nights).” These are five prayers.
Allah, the most High, said about this issue, “Perform prayer on both ends of the day, [which is sunset and morning], and in a part of the night.” (11:114) This is al-Isha’ prayer. He, the most High has said, “Preserve prayer especially the middle prayer.” (2:239) Middle prayer is al-Dhuhr prayer which is the first prayer the Messenger of Allah (sw) performed. It is in the middle of two prayers during the day, the morning and al-‘Asr prayer. In some of the recitations it is said, “Preserve Salawat (prayers) and the middle prayer.” Middle prayer is al-‘Asr prayer.’
About the words of Allah, “Stand up praying before Allah,” he (the Imam) said, ‘This verse was revealed on Friday when the Messenger of Allah (sw) was on a journey. The Messenger of Allah (sw) prayed before Allah on Friday and left it in that condition during a journey or when at home. For those at home he added two Rak‘ats. He reduced two Rak‘at which the Holy Prophet (sw) had added on Friday for people when at home, because of the two sermons which a prayer leader delivers. Those who perform Friday prayer without congregation, they perform it four Rak‘at like al-Dhuhr prayer in other days.’”