When was prayer made obligatory? How did the Muslims pray before prayer was made obligatory?

Al-Bukhaari (349) and Muslim (162)
narrated from Anas ibn Maalik (may Allah be pleased with him) the famous
hadeeth of the Isra’ (Prophet’s Night Journey) in which it is reported
that the Messenger of Allah (blessings and peace of Allah be upon him)
said:

“Then Allah revealed what He revealed to me,
and enjoined fifty prayers on me every day and night. I came back down to
Moosa (blessings and peace of Allah be upon him) and he said: What did your
Lord enjoin upon your ummah? I said: Fifty prayers. He said: Go back to your
Lord and ask Him to reduce it… I kept going back and forth between my
Lord, may He be blessed and exalted, and Moosa (peace be upon him), until He
said: ‘O Muhammad, they are five prayers each day and night, for every
prayer there will be a tenfold (reward), and that is fifty prayers.” 

The scholars are unanimously agreed that the
five daily prayers were not made obligatory until this night. See the answer
to question no. 143111

See also Fath al-Baari by Ibn Rajab
(2/104). 

Al-Haafiz Ibn Katheer (may Allah have mercy
on him) said: 

On the night of the Isra’, one and a half
years before the Hijrah, Allah enjoined upon His Messenger (blessings and
peace of Allah be upon him) the five daily prayers, and explained that there
were conditions and essential parts and other matters having to do with
them, bit by bit. 

Tafseer Ibn Katheer (7/164). 

Then Jibreel (peace be upon him) came down
and taught the Prophet (blessings and peace of Allah be upon him) the times
of the prayers: 

Al-Bukhaari (522) and Muslim (611) narrated
from Ibn Shihaab that ‘Umar ibn ‘Abd al-‘Azeez delayed the prayer one day.
‘Urwah ibn az-Zubayr entered upon him and told him that al-Mugheerah ibn
Shu‘bah delayed the prayer one day when he was in Kufah, and Abu Mas‘ood
al-Ansaari entered upon him and said: What is this, O Mugheerah? Do you not
know that Jibreel came down and prayed, and the Messenger of Allah
(blessings and peace of Allah be upon him) prayed, then he prayed and the
Messenger of Allah (blessings and peace of Allah be upon him) prayed, then
he prayed and the Messenger of Allah (blessings and peace of Allah be upon
him) prayed, then he prayed and the Messenger of Allah (blessings and peace
of Allah be upon him) prayed, then he prayed and the Messenger of Allah
(blessings and peace of Allah be upon him) prayed. Then he said: This is
what has been enjoined upon me. ‘Umar said to ‘Urwah: Think what you are
narrating, O ‘Urwah! Is Jibreel the one who taught the Messenger of Allah
(blessings and peace of Allah be upon him) the times of the prayers? ‘Urwah
said: That is what Basheer ibn Abi Mas‘ood used to narrate from his father. 

An-Nasaa’i (526) narrated that Jaabir ibn
‘Abdullah (may Allah be pleased with him) said: Jibreel (peace be upon him)
came to the Prophet (blessings and peace of Allah be upon him) when the sun
had passed its zenith and said: Get up, O Muhammad. That was when the sun
had passed the meridian. Then he waited until the (length of) a man’s shadow
was equal to his height, then he came to him for ‘Asr and said: Get up, O
Muhammad, and pray ‘Asr. Then he waited until the sun set, then he came to
him and said: Get up and pray Maghrib. So he got up and prayed it when the
sun had set fully. Then he waited until the twilight had disappeared, then
he came and said: Get up and pray ‘Isha’, so he got up and prayed it… In
this hadeeth it says: He (i.e., Jibreel) said: The period between two of
these two times is the time to pray.

Classed as saheeh by al-Albaani in Saheeh
an-Nasaa’i. 

‘Abd ar-Razzaaq narrated in his Musannaf
(1773) and Ibn Ishaaq narrated in his Seerah – as it says in Fath al-Baari
(2/286) – that this happened on the morning after the night in which prayer
was made obligatory. 

Al-Qurtubi (may Allah have mercy on him)
said: 

(The scholars) did not differ concerning the
fact that Jibreel (peace be upon him) came down on the morning following the
night of the Isra’, when the sun passed its zenith, and taught the Prophet
(blessings and peace of Allah be upon him) the prayer and its timings. End
quote. 

Shaykh al-Islam Ibn Taymiyah (may Allah have
mercy on him) said: 

Jibreel’s explanation of the timings of the
prayer came on the morning following the night of the Isra’. End quote. 

Sharh al-‘Umdah (4/148) 

When the five daily prayers were first made
obligatory, each prayer was two rak‘ahs, then after the Hijrah, that was
confirmed in the case of travel, and two rak‘ahs were added for those who
are not travelling, except Maghrib, which remained as it was. Al-Bukhaari
(3935) and Muslim (685) narrated that ‘Aa’ishah (may Allah be pleased with
her) said: Prayer was enjoined with two rak‘ahs, then when the Prophet
(blessings and peace of Allah be upon him) migrated, it was enjoined with
four rak‘ahs, but prayer whilst travelling remained as it had originally
been. 

The Prophet (blessings and peace of Allah be
upon him) and his Companions used to pray before the five daily prayers were
made obligatory. 

It says in al-Mawsoo‘ah al-Fiqhiyyah
(27/52-53): 

Prayer was originally enjoined in Makkah at
the beginning of Islam, because there are Makkan verses that were revealed
at the beginning of the Prophet’s mission that encourage praying. As for the
five daily prayers in the form that is well-known, they were made obligatory
on the night of the Isra’ and Mi‘raaj. End quote. 

See also the answer to question no.
143111 

Some of the scholars are of the view that
prayer was made obligatory at the beginning, two rak‘ahs in the morning and
two rak‘ahs in the afternoon. 

Al-Haafiz (may Allah have mercy on him) said
in al-Fath: 

A number of scholars are of the view that
before the Isra’ there was no obligatory prescribed prayer, but prayers were
offered at night, without any number of rak‘ahs being specified. Al-Harbi
was of the view that prayer was made obligatory, two rak‘ahs in the morning
and two rak‘ahs in the afternoon. Ash-Shaafa‘i narrated from some of the
scholars that prayer at night was obligatory, then it was abrogated by the
verse (interpretation of the meaning): “So, recite you of the Quran as much
as may be easy for you” [al-Muzammil 73:20]. So it became obligatory to pray
for part of the night. Then that was abrogated by the five daily prayers.
End quote. 

He also said: 

Before the Isra’, the Prophet (blessings and
peace of Allah be upon him) definitely used to pray, as did his companions,
but there is a scholarly difference of opinion as to whether any kind of
prayer was made obligatory before the five daily prayers or not. It was said
that what was obligatory at first was prayer before sunrise and prayer
before sunset. The evidence for that is the verse in which Allah, may He be
exalted, says (interpretation of the meaning): “and glorify the praises of
your Lord before the rising of the sun, and before its setting” [Ta-Ha
20:130], and similar verses. End quote. 

See also Tafseer Ibn ‘Atiyyah (1/204);
at-Tahreer wa’t-Tanweer by Ibn ‘Ashoor (24/75). 

And Allah knows best.