The Adhan (Call to Prayer); “Hayya ‘ala Khayril ‘Amal” (Come to the Best of Deeds)
The entire adhan (call to prayer) was taught to the Prophet Muhammad by Allah on the night he ascended to Heaven, and the prayers were made obligatory on him that same night. The original adhan taught to him contained the phrase “hayya ‘ala khayril ‘amal” (come to the best of deeds); however, at the time the Islamic state was expanding, the second caliph, ‘Umar ibn al-Khattab thought that this phrase would discourage people from performing jihad (defense fighting) and thus ordered it to be removed from the adhan. Imam Muslim narrates, on the authority of Ibn Mas‘ud that the Prophet had commanded the Muslims to say in the adhan and iqaama (the call that signals the beginning of the prayer) “hayya ‘ala khayril ‘amal,” but once ‘Umar assumed authority he dropped that phrase. He also says that ‘Ali ibn Abi Talib and his followers, as well as, ‘Abdullah, the son of ‘Umar did not drop this phrase.
‘Umar ibn al-Khattab has been narrated to have said, “O people, three things existed during the time of the Messenger of Allah that I prohibit and make unlawful and will punish for, they are: mut‘at al-hajj, mut‘at al-nisa, and ‘hayya ‘ala khayr al-‘amal.’ (the Mut’ah of the Hajj, mut’ah of the woman and ‘hasten towards the best of deeds’)”
Malik ibn Anas narrates the story of how ‘hayya ‘ala khayr al-‘amal’ (Hasten towards the best of deeds) was replaced by “al-salat khayrun min al-nawm” (The prayers are better than sleep.) Anas said, “The mu’adhdhin (the person making the call to prayer) came to ‘Umar ibn al-Khattab to announce the morning prayers and found him asleep, so he said to him, ‘al-salat khayrun min al-nawm’ (prayer is better than sleep). ‘Umar liked this sentence very much, so he ordered that it be included in the adhan for the morning prayers.” Imam Muslim and Abu Dawud also concur that this sentence was not part of the adhan during the time of the Prophet, and Tirmidhi asserts that ‘Umar was the one who added it.
Some people may wonder why the Shi‘a, in the adhan, include: “Ashhadu anna ‘Ali`yan waliuAllah” (“I testify that ‘Ali is the close friend of Allah”) after the first two testimonies. All the Shi‘a jurists and scholars have a consensus that this sentence is not an obligatory part of the adhan; nonetheless, saying it is a tradition. However, if anyone says it in the adhan, believing it to be obligatory, then his or her adhan will become void. The Shi‘as believe it began during the time of the Prophet, on the day of Ghadir after he appointed Imam ‘Ali as his successor, during which the Muslims paid their allegiance to Imam ‘Ali, and Abu Dharr al-Ghifari recited the adhan and added the phrase: “Ashhadu anna ‘Ali`yan wali Allah.” Afterwards, the Muslims came to the Prophet and said that they had heard something new in the adhan. When the Prophet asked what they had heard, they replied, we heard the phrase, “Ashhadu anna ‘Aliyan wali Allah” in the adhan. The Prophet asked them whether they had not just acknowledged this same phrase to Imam ‘Ali when they gave their allegiance (bay’ah) to him.
 Al-Muttaqi al-Hindi, Kanz al-Ummal, Hadith 397, Vol. 6; al-Hakim, al-Mustadrak, Vol. 3; 1
 Sahih Muslim, Vol. 1, 48
 Al-Sirah al-Halabiyyah, Vol. 4, 56
 Sharh al-Tajrid; Musnad Ahmad ibn Hanbal, Vol. 1, 49
 Malik ibn Anas, Kitab al-Muwatta’, Ch. “Adhan”
 Sunan al-Tirmidhi, Vol. 1, 64