Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Ibn Mughaffal Muzani (may Allah be pleased with him), he states that the Noble Prophet (peace and blessings be upon him) said: "When you find the Imam in the state of prostration, then perform prostration with him; if you find him in the state of bowing, then bow with him; and if you find him standing, then begin standing with him. But do not count the prostrations unless you have caught the bowing."