Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "When the imam says 'Allahu akbar' (Allah is the Greatest), then you say 'Allahu akbar'; and when he says 'Sami' Allahu liman hamidah' (Allah hears the one who praises Him), then you say 'Rabbana wa lakal-hamd' (Our Lord, and to You belongs all praise)." This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it with these words. It contains an important Sunnah: that the follower should wait until the imam completes the takbir and should not say the takbir together with him (at the same moment).