Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ سَأَلَ اَلْفُضَيْلُ بْنُ يَسَارٍ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنْ رَجُلٍ صَلَّى مَعَ إِمَامٍ يَأْتَمُّ بِهِ ثُمَّ رَفَعَ رَأْسَهُ مِنَ اَلسُّجُودِ قَبْلَ أَنْ يَرْفَعَ اَلْإِمَامُ رَأْسَهُ مِنَ اَلسُّجُودِ قَالَ «فَلْيَسْجُدْ»
Al-Fudayl ibn Yasar asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام about a man who prays behind an Imam whom he is following and raises his head from prostration (Sujood) before the Imam raises his head. Imam عليه السلام said: "He should prostrate again."
- Ṣaḥīḥ al-Faqīh, p. 57