Some of the practices that Umar is best known for changing are the adhan (call to prayer), tarawih prayer, prayers for the deceased, and the laws of divorce which we will explain in detail:
Adhan (Call to Prayer)
Initially, “as-salatu khayrun min an-nawm (prayer is better than sleep)” was not part of the adhan. It came about one morning when Umar’s servant came to wake him for prayer by calling to him, “as-salatu khayrun min annawm.” Approving of that phrase, Umar instructed the muadhdhin (a person who performs the call to prayer) to include it in the adhan from then on.
History of the Adhan
A majority of the Sunni commentators maintain that the Prophet learned the adhan from a companion named Abdullah b. Zayd. Narrators such as al-Tirmidhi, Abu Dawud, and Ibn Majah say that the Prophet asked his companions how he should inform people about the time for prayer. Some said he should use a banner, others said a horn, and others said a bell, like the Christians. According to these narrators, it is reported that on that night, Abdullah b. Zayd had the following strange dream:
Abdullah b. Zayd saw a man carrying a bell in his hand. He asked if he could buy the bell, and the man asked him what he wanted it for. Abdullah b. Zayd said, “I want to use it to call the people to prayer.” The man replied, “I can teach you something better than ringing the bell,” and then he taught Abdullah b. Zayd the adhan.
According to these commentators, in the morning Abdullah b. Zayd told the Prophet his dream. The Prophet replied, “This is a true dream that you have seen. Come with me to Bilal – teach him what you have seen in the dream and let Bilal learn the adhan.” The story concludes that when Umar heard the adhan, he told the Prophet, “I swear, I saw the same dream,” and the Prophet said, “praise be to Allah.”
Clearly, this story cannot be true for surely Allah, the Merciful, the Compassionate to humankind, would not provide detailed revelation and then omit an important practice such as the adhan.
Nonetheless, there exists another explanation, which is accepted by Shia scholars:
Gabriel came to the Prophet while the Prophet was resting on Ali. Gabriel read the adhan to them. To maintain this, the Prophet turned to Ali and said, “Did you hear the adhan?” Ali said, “Yes.” The Prophet said, “Did you memorize it?” Ali said, “Yes.” The Prophet said, “Then call Bilal; let us teach him the adhan.” So they called Bilal and taught him the adhan.
An important point to note is that because the instructions came from angel Gabriel, the adhan must be considered as being part of the revelation, and not a whimsical dream seen by one of the companions.
Prayer without Taharah (Ritual Purity)
Al-Bukhari narrates that one day a man came to Umar and said, “I am in a state of janabah (ritual impurity) and I cannot find water.” Umar told him, “Do not pray.” Ammar b. Yasir, who was sitting there retorted:
Do you not remember that you and I were in a battalion going to the battles, and both of us woke up and found ourselves in a state of ritual impurity, and we did not find water? So you did not pray, but I did the tayammum (ritual purification by means of dust) in the dust. We mentioned this to the Prophet, and the Prophet said that you should have put your hands in the dust and performed the tayammum. 
Primarily, the Qur’an prescribes the tayammum for situations when water cannot be found. (c. 4:43) Therefore, most of the Sunni schools of thought follow the Qur’an in this regard. Only the Hanafi school follows Umar’s opinion and indicates that tayammum is permissible only while traveling or ill, but in all other cases, a person who cannot find water for taharah should not pray.
Al-Bukhari narrates from Abdullah b. Abd al-Qari:
I went with Umar b. al-Khattab during his period of caliphate one night in the Month of Ramadhan to the mosque. We saw the people praying scattered, not together. Umar said, “It is best if these people can pray together, and there is only one who leads the prayers.” So he gathered them and appointed Ubay b. Ka’ab to lead the prayers. Another night, I went with Umar to the mosque and saw that the people were praying together, organized, and Umar said, “What an excellent innovation (niam al-bidah hadhihi).”
Prayers for the Deceased
Although narrators like Ahmad b. al-Hanbal, al-Muslim, and al-Nisa’i all relate that the Prophet read prayers over the deceased with five takbiraat (uttering “Allahu akbar”); however, Umar reduced the number of takbiraat in prayers over the deceased from five to four.
Three Divorces in One Session
According to the Holy Qur’an (c. 2:229), a married couple may divorce each other three times before they are no longer allowed to remarry each other. Thus, divorce must be declared and its rulings applied on three separate periods; the pronouncement of the three divorces cannot be declared in one declaration. The Holy Prophet explained:
A man came to the Prophet and told him, “I divorced my wife.” The Prophet asked, “How did you divorce her?” He said, “Three times in one session.” The Prophet said, “That divorce is considered only one divorce. It cannot be considered three divorces, so you may take your wife back.”
However, historians say that divorce became more prevalent during the time of the second caliph; thus to make divorce easier, Umar allowed men to read all three pronouncements of divorce at one time.
Other Actions performed by Umar
Umar also did following:
- Prevented the death announcement of the Prophet
- Objected to the Treaty of Hudaybiyyah
- Refused to join the dispatch of Usama as the Prophet commanded him to do so just before his death
- Prevented the Prophet from narrating his will
- He was the first person to give allegiance to Abu Bakr at Saqifah
- Offered two options to Ali and Fatima al-Zahra – pay allegiance to Abu Bakr or face the consequences
- Appointed Mu’awiyah as governor of Syria
- Appointed Abu Huraira as governor of Bahrain and then accused him of theft and lying about the statements of the Prophet
- Allocated different salaries to different groups of people, introducing discrimination into the financial system
- Permitted wiping over one’s socks in wudhu instead of removing them and performing it on the bare feet;
- Prevented people from mourning over the dead
- Forced people who had taken the names of prophets (as their first names) to change their names
- Required all men to offer the same amount of mahr (marriage gift) to all women
 In the Sunni tradition, tarawih is done as a congregational prayer during the nights of the Month of Ramadhan, while the Shia perform it individually.
 Jalal al-Deen al-Suyuti, footnoted in Tanweer al-Hawalik, referencing Muwatta Malik; Sunan al-Tirmidhi, 1:64
 Wasail al-Shiah, 4:612
 Prophet Muhammad said, “Ali, you can see all that I can see, and you can hear all that I can hear; except that you are not a prophet but a vicegerent and you are virtually on the path of virtue.” Nahjul Balaghah, sermon 192
 Sunan al-Nisa’i, 1:169; Sunan Ibn Majah, 1:188; Al-Bayhaqi, Al-Sunan al-Kubaa, 1:209; Tafseer Ibn Katheer, 4:505; Ibn Qudaamah, Al-Mughni, 1:234; Ibn Rushd, Al-Bidayah wal-Nihayah, 1:63
 Sahih al-Bukaari, 3:58; Tarikh al-Madinah al-Munawarah, 2:713; Al-Riyadh al-Nadhirah, 1:309; Tarikh al-Yaqubi, 2:114
 Musnad Ahmad b. Hanbal, 4:370; Sahih al-Muslim, Baab al-Salat ala al-Qabr; Kitab al-Janazah
 Al-Suyuti, Tarikh al-Khulafah, p. 137
 Al-Jassas, Ahkam al-Qur’an, 1:378
 Sirat b. Ishaq, 2:191
 Sahih al-Muslim, Chapter of Talaq al-Thalath, 1:575; Musnad Ahmad b. Hanbal, 1:314; Al-Bayhaqi, 7:336
 Umdat al-Qari, 4:87; Sahih al-Bukhari, 2:102; Sahih al-Muslim, 2:238
 Tabaqat b. Sa’d, 5:51; Umdat al-Qari, 7:143