It is narrated from Zayd bin Wahb: I went out with Abdullah bin Mas'ud (may Allah be pleased with him) from his house towards the mosque. When we reached the middle of the mosque, the imam bowed. Abdullah (may Allah be pleased with him) said the takbir and bowed, and I bowed with him. Then we walked in the state of bowing until we reached the row just as the people raised their heads. When the imam completed the prayer and said the salam, I stood up, because I thought that I had not caught the rak'ah. Abdullah bin Mas'ud (may Allah be pleased with him) took hold of my hand and sat me down, then said: You have caught the rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2587