Al-Kāfi - Volume 3 > Recitations in Salah (prayer)
Hadith #5

5ـ عَلِيٌّ عَنْ أَبِيهِ عَنْ عَبْدِ الله بْنِ الْمُغِيرَةِ عَنْ جَمِيلٍ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ الْسَّلام) قَالَ إِذَا كُنْتَ خَلْفَ إِمَامٍ فَقَرَأَ الْحَمْدَ وَفَرَغَ مِنْ قِرَاءَتِهَا فَقُلْ أَنْتَ الْحَمْدُ لله رَبِّ الْعَالَمِينَ وَلاَ تَقُلْ آمِينَ.

5. Ali has narrated from his father from ‘Abd Allah ibn al-Mughirah from Jamil who has said the following: “Abu ‘Abd Allah, ‘Alayhi al-Salam, has said, ‘If you are behind an Imam when he completes reading al-Hamd you say, ‘all praise belongs to Allah, Cherisher of the worlds,’ and do not say ‘Amen.’” (Ahadith 4-5 are best explained in the form of fatwa.)