When were the five daily prayers made obligatory?

What was made obligatory on the night of the Miʿraj was the five daily prayers. However, prayer was in practice from the very beginning of the Prophetic mission. It has been established that the Prophet (peace and blessings be upon him) prayed in the earliest days of the mission.

ʿAffif al-Kindi (may Allah be pleased with him) narrated, “I was a merchant, and I came for pilgrimage. I approached al-ʿAbbas Ibn ʿAbd al-Muttalib (may Allah be pleased with him) to purchase some goods, for he was a merchant. By Allah, while I was with him at Mina, a man came out of a nearby tent, looked at the sun, and when he saw that it had declined, he stood and prayed. Then a woman came out of the same tent and stood behind him, praying. Then a young boy, who had just reached the age of puberty, came out of the tent and stood beside him, praying.

I asked al-Abbas, “Who is this, O Abbas?”

He replied, “This is Muhammad Ibn Abdullah Ibn Abdul Muttalib, my nephew.”

I then said, “And who is this woman?”

He said, “This is his wife, Khadija Bint Khuwaylid.”

I then asked, “Who is the boy?”

He answered, “He is Ali Ibn Abu Talib, his cousin.”

I asked him, “What is he doing?”

He replied, “He is praying; he claims to be a prophet. No one has followed him except his wife and this young boy, his cousin. He claims that he will conquer the lands of Chosroes and Caesar.”

Affif (may Allah be pleased with him) who was the cousin of al-Ashʿath Ibn Qays (may Allah be pleased with him), used to say (he embraced Islam later, and his Islam was sound), “If Allah had granted me Islam on that day, I would have been the third to join alongside ʿAli Ibn Abu Talib (may Allah be pleased with him)” (recorded by Ahmad in his Musnad and by al-Tabarani in Al-Mu’jam Al-Kabir).

The scholar Ibn Hajar commented in Fatḥ al-Bari (7:203), saying, “There is a difference of opinion regarding when prayer was first made obligatory. It was said it was at the beginning of the [prophetic] mission, with the prayers consisting of two rakʿahs in the morning and two in the evening. What was made obligatory on the night of Isra` (the Night journey) was the five daily prayers.

As previously mentioned in the biography of Khadija (may Allah be pleased with her), in the discussion on the hadith of ʿAʾisha (may Allah be pleased with her) regarding the beginning of creation, that ʿAʾisha affirmed that Khadija passed away before prayer was made obligatory. The relied-upon opinion is that those who said ‘after prayer was made obligatory’ meant the prayers that preceded the obligation of the five prayers, if that can be established. ʿAʾisha’s words that ‘she passed away before prayer was made obligatory’ refers to the five daily prayers. Thus, the two statement can be reconciled in this manner, and it follows that she [Khadija] passed away before the Isra`.”

And Allah Almighty knows best.