(a) It is narrated from Ibn Awn that Muhammad bin Sirin (may Allah have mercy on him) said: When the imam says 'Sami'allahu liman hamidah' (Allah has heard the one who praised Him), those behind him should say: 'Sami'allahu liman hamidah' (Allah has heard the one who praised Him).
(b) It is narrated from Abu Burdah bin Abi Musa that he used to say behind the imam 'Sami'allahu liman hamidah' (Allah has heard the one who praised Him).
(c) Ata' said: The preferred practice in my view is that he combines the two along with the imam.
Two weak (da'if) hadiths have also been narrated on this; I have recorded them in al-Khilaf.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2616