Muhammad ibn Yaqub narrated from Muhammad ibn Yahya, from Abdillah ibn Muhammad ibn Isa, from Ali ibn al-Hakam, from Aban, from Abd al-Rahman ibn Abi Abdillah, from Abu Abdillah عليه السلام, who said: Imam al-Sadiq عليه السلام said: “If you enter the mosque and the imam (of prayer) is in rukuh, and you think that if you walk to him, he will raise his head before you catch him, then say the takbir and bow. If he raises his head (before you reach him), then prostrate where you are. When he stands, join the row; and if he sits, then sit where you are. When he stands again, join the row.”