Abu Ja’far (as) has said: ‘It is upon you to have enthusiasm in your Prayers, for what is accepted from you (your prayers) is from what your heart accepts from it. And during it, do not fiddle with your hands, nor with your head, nor with your beard, nor speak with yourself, nor yawn, nor stretch your muscles, nor blaspheme. But rather, these are the action of the Majoos (Magians). And when you are free from the recitation (of surah al-Fatiha), do not be saying: “Ameen”, for its place you may say: ‘All Praise is for Allah, Lord of the Worlds’ (الحمد الله رب العالمين).
And he (as) said: ‘Neither veil yourself, nor display shyness, nor fall upon your feet, nor stretch your arms out upon the ground, nor pop (crack) your fingers, for it (all of) these are faults in the Prayer’.
And he (as) said: ‘Do not stand to the Prayer slouching, nor shuffling, nor sluggishly (lazily), for these are from the hypocrisy, and Allah Mighty and Majestic has Forbidden the Believers that they should be standing to the Prayer and they are intoxicated – Meaning from the sleep – and Said for the hypocrites “and when they stand up for the Prayer they stand up sluggishly; they do it only to be seen of men and do not remember Allah except for a little” (4:142).’
Sanad: Muhammad Bin Ali Majaylawiya narrated to us, from Ali Bin Ibrahim, from his father, from Hamaad, from Hareyz, from Zarara who said the above hadith
Source: `ilal al-Shara’i`, Sh. al-Sadooq, Volume 2, Chapter 74, Hadith #1