Obligation of the first takbir

It was narrated from Abu Hurairah that:

The Messenger of Allah ﷺ entered the Masjid, then a man entered and prayed, then he came and greeted the Messenger of Allah ﷺ with Salam. The Messenger of Allah ﷺ returned his greeting and said: Go back and pray, for you have not prayed." So he went back and prayed as he has prayed before, then he came to the Prophet ﷺ and greeted him with Salam, and the Messenger of Allah ﷺ said to him: "Wa alaika as-salam (and upon you be peace). Go back and pray for you have not prayed." He did that three times, then the man said: "By the One Who sent you with the truth, I cannot do any better than that; teach me." He said: "When you stand to pray, say the Takbir, then recite whatever is easy for you of Quran. Then bow until you have tranquility in your bowing, then stand up until you are standing straight. Then prostrate until you have tranquility in your prostration, then sit up until you have tranquility in your sitting. Then do that throughout your entire prayer."

أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا يَحْيَى، قَالَ حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ، قَالَ حَدَّثَنِي سَعِيدُ بْنُ أَبِي سَعِيدٍ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ ﷺ دَخَلَ الْمَسْجِدَ فَدَخَلَ رَجُلٌ فَصَلَّى ثُمَّ جَاءَ فَسَلَّمَ عَلَى رَسُولِ اللَّهِ ﷺ فَرَدَّ عَلَيْهِ رَسُولُ اللَّهِ ﷺ وَقَالَ " ارْجِعْ فَصَلِّ فَإِنَّكَ لَمْ تُصَلِّ " . فَرَجَعَ فَصَلَّى كَمَا صَلَّى ثُمَّ جَاءَ إِلَى النَّبِيِّ ﷺ فَسَلَّمَ عَلَيْهِ فَقَالَ لَهُ رَسُولُ اللَّهِ ﷺ " وَعَلَيْكَ السَّلاَمُ ارْجِعْ فَصَلِّ فَإِنَّكَ لَمْ تُصَلِّ " . فَعَلَ ذَلِكَ ثَلاَثَ مَرَّاتٍ فَقَالَ الرَّجُلُ وَالَّذِي بَعَثَكَ بِالْحَقِّ مَا أُحْسِنُ غَيْرَ هَذَا فَعَلِّمْنِي . قَالَ " إِذَا قُمْتَ إِلَى الصَّلاَةِ فَكَبِّرْ ثُمَّ اقْرَأْ مَا تَيَسَّرَ مَعَكَ مِنَ الْقُرْآنِ ثُمَّ ارْكَعْ حَتَّى تَطْمَئِنَّ رَاكِعًا ثُمَّ ارْفَعْ حَتَّى تَعْتَدِلَ قَائِمًا ثُمَّ اسْجُدْ حَتَّى تَطْمَئِنَّ سَاجِدًا ثُمَّ ارْفَعْ حَتَّى تَطْمَئِنَّ جَالِسًا ثُمَّ افْعَلْ ذَلِكَ فِي صَلاَتِكَ كُلِّهَا " .