✅ Answer: Pay attention to the following narrations:

1⃣ Imam Baqir (peace be upon him) said in a hadith to Zurarah:

📚 Kafi, Vol. 3, p. 335

وَ إِذَا قَعَدْتَ فِي تَشَهُّدِكَ فَأَلْصِقْ رُكْبَتَيْكَ بِالْأَرْضِ وَ فَرِّجْ بَيْنَهُمَا شَيْئاً وَ لْيَكُنْ ظَاهِرُ قَدَمِكَ الْيُسْرَى عَلَى الْأَرْضِ وَ ظَاهِرُ قَدَمِكَ الْيُمْنَى عَلَى بَاطِنِ قَدَمِكَ الْيُسْرَى وَ أَلْيَتَاكَ عَلَى الْأَرْضِ وَ أَطْرَافُ إِبْهَامِكَ الْيُمْنَى عَلَى الْأَرْضِ وَ إِيَّاكَ وَ الْقُعُودَ عَلَى قَدَمَيْكَ فَتَتَأَذَّى بِذَلِكَ وَ لَا تَكُونُ قَاعِداً عَلَى الْأَرْضِ فَيَكُونَ إِنَّمَا قَعَدَ بَعْضُكَ عَلَى بَعْضٍ فَلَا تَصْبِرَ لِلتَّشَهُّدِ وَ الدُّعَاءِ.

The quality of sitting for men during Tashahhud is described in detail in this hadith:

1. Press both knees to the ground.

2. Keep a slight distance between both knees.

3. Place the top of the left foot on the ground.

4. Have the top of the right foot on the sole of the left foot.

5. Both buttocks should be on the ground.

6. The sides and edges of the big toe of the right foot should be on the ground.

✅ The details above indicate the quality of sitting during Tashahhud as mentioned in this narration.

Additionally, in a hadith narrated by Hammad ibn 'Isa in the book Kafi, Vol. 3, p. 312, the narrator describes the Imam Sadiq (peace be upon him) sitting between the two prostrations:

1. He sat on his left thigh.

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

2. He placed the top of his right foot on the sole of his left foot and said: "I seek forgiveness from Allah, my Lord, and I repent to Him." Then, while still sitting, he said the Takbir and went into the second prostration.

✅ The way women sit during Tashahhud is different and will be mentioned separately, إن شاءالله (if Allah wills).

اللَّهُمَّ صَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ