✅ Answer:

Please pay attention to the following narration:

1 Hammad ibn Isa says in his description of the prayer of Imam Sadiq (peace be upon him):

ثُمَّ اسْتَوَى قَائِماً فَلَمَّا اسْتَمْكَنَ مِنَ الْقِيَامِ قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ- ثُمَّ كَبَّرَ وَ هُوَ قَائِمٌ وَ رَفَعَ يَدَيْهِ حِيَالَ وَجْهِهِ وَ سَجَدَ وَ وَضَعَ يَدَيْهِ إِلَى الْأَرْضِ قَبْلَ رُكْبَتَيْهِ فَقَالَ سُبْحَانَ رَبِّيَ الْأَعْلَى وَ بِحَمْدِهِ ثَلَاثَ مَرَّاتٍ وَ لَمْ يَضَعْ شَيْئاً مِنْ بَدَنِهِ عَلَى شَيْ ءٍ مِنْهُ وَ سَجَدَ عَلَى ثَمَانِيَةِ أَعْظُمٍ الْجَبْهَةِ وَ الْكَفَّيْنِ وَ عَيْنَيِ الرُّكْبَتَيْنِ وَ أَنَامِلِ إِبْهَامَيِ الرِّجْلَيْنِ وَ الْأَنْفِ فَهَذِهِ السَّبْعَةُ فَرْضٌ وَ وَضْعُ الْأَنْفِ عَلَى الْأَرْضِ سُنَّةٌ وَ هُوَ الْإِرْغَامُ ثُمَّ رَفَعَ رَأْسَهُ مِنَ السُّجُودِ فَلَمَّا اسْتَوَى جَالِساً قَالَ اللَّهُ أَكْبَرُ- ثُمَّ قَعَدَ عَلَى جَانِبِهِ الْأَيْسَرِ وَ وَضَعَ ظَاهِرَ قَدَمِهِ الْيُمْنَى عَلَى بَاطِنِ قَدَمِهِ الْيُسْرَى وَ قَالَ أَسْتَغْفِرُ اللَّهَ رَبِّي وَ أَتُوبُ إِلَيْهِ- ثُمَّ كَبَّرَ وَ هُوَ جَالِسٌ وَ سَجَدَ الثَّانِيَةَ وَ قَالَ كَمَا قَالَ فِي الْأُولَى...

📚 Al-Faqih, Vol. 1, p. 300

Then Imam Sadiq (peace be upon him) stood up straight after the bowing. When he was completely settled in standing, he said:

سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ

'Sami' Allahu liman hamidah' (Allah hears the one who praises Him).

سُبْحَانَ رَبِّيَ الْأَعْلَى وَ بِحَمْدِهِ

Then, while standing, he said 'Allahu Akbar' and raised his hands to his face and went into prostration. He placed his hands on the ground before his knees and said three times:

أَسْتَغْفِرُ اللَّهَ رَبِّي وَ أَتُوبُ

'Subhana Rabbi al-a'la wa bi hamdih' (Glory be to my Lord, the Most High, and praise be to Him).

2 عَنْ عَلِيِّ بْنِ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنْ حَمَّادِ بْنِ عِيسَى عَنْ حَرِيزٍ عَنْ زُرَارَةَ قَالَ: قَالَ أَبُو جَعْفَرٍ إِذَا أَرَدْتَ أَنْ تَرْكَعَ وَ تَسْجُدَ فَارْفَعْ يَدَيْكَ وَ كَبِّرْ ثُمَّ ارْكَعْ وَ اسْجُدْ.

And he did not place any part of his body on another and prostrated on eight bones, which were: the forehead, the two palms, the two kneecaps, and the two big toes of the feet, of which seven are obligatory and placing the nose on the ground is a tradition and is 'irgham.' Then he raised his head from prostration, and when he was completely upright in a sitting position, he said: 'Allahu Akbar' and sat on his left (thigh) and placed the top of his right foot on the bottom of his left foot and said:

'Astaghfirullaha Rabbi wa atubu ilayh' (I seek forgiveness from Allah, my Lord, and I repent to Him).

Then, while in that sitting position, he said 'Allahu Akbar' and went into the second prostration and said the same thing he said in the first prostration...

📚 Al-Kafi, Vol. 3, p. 320

Imam Baqir (peace be upon him) said: 'When you want to go into bowing and prostration, first raise your hands and say 'Allahu Akbar' and then go into bowing and prostration!'

✅ It is clear that the Takbirs for prostration are four:

The first Takbir is said while standing after 'Sami' Allahu liman hamidah.'

The second Takbir is said after the first prostration and while the person is sitting completely upright.

اللَّهُمَّ صَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ

وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ

The third Takbir is also said in the same sitting position between the two prostrations before going into the second prostration.