It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) that he said: A man stood behind the Prophet of Allah (peace and blessings be upon him) while he (peace and blessings be upon him) was praying, and he said: "Allahu akbaru kabira (Allah is the Greatest, greatly)" - (then the narrator related the complete hadith).
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18278