The first prayer in which the Prophet (peace and blessings of Allah be upon him) bowed

Firstly: 

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Secondly: 

Some of the scholars have stated that the first prayer in which the Prophet (peace and blessings of Allah be upon him) bowed was ‘Asr prayer following the Isra’ and Mi’raaj (the Prophet’s Night Journey and Ascent into heaven).

 Al-Ramli al-Shaafa’i said in Nihaayat al-Muhtaaj (1/396): 

Bowing is one of the unique characteristics of this ummah. The first prayer in which the Prophet (peace and blessings of Allah be upon him) bowed was ‘Asr prayer on the day after the Isra’. Al-Suyooti quoted as evidence for that the fact that it is narrated that the Prophet (peace and blessings of Allah be upon him) prayed Zuhr on that day without bowing, and that before that he had prayed that night in like manner. Were it not that bowing is one of the unique characteristics of this ummah, he would have done it in the prayers that he offered before the Isra’ and in Zuhr prayer of the day after. 

There is a hadeeth concerning that which was narrated from the Prophet (peace and blessings of Allah be upon him), but there is some weakness in it. 

Al-Bazzaar and al-Tabaraani (in al-Awsat) narrated that ‘Ali (may Allah be pleased with him) said: The first prayer in which we bowed was ‘Asr. I said: “O Messenger of Allah, what is this?” He said: “I was commanded to do it.” 

Al-Haythami said in Majma’ al-Zawaa’id (1/293): 

This was narrated by al-Bazzaar and by al-Tabaraani in al-Awsat, but its isnaad includes Abu ‘Abd al-Raheem – if he is Khaalid ibn Yazeed, then he is thiqah (trustworthy) and is one of the men of saheeh, and I did not find any other Abu ‘Abd al-Raheem in the men of books apart from him, and I did not find Abu ‘Abd al-Raheem in al-Meezaan, and he is majhool (unknown). 

The hadeeth is mentioned in Kanz al-‘Ummaal (21779), where it is suggested that it is weak. 

And Allah knows best.