Reference: Fataawa tata’allaq bi-Jamaa’atil-Masjid, Question 37, Page 35
Majmoo’ Fataawa, Volume 4, Page 426
Question: If the imaam goes into the rukoo’ position before me, is it permissible for me to say ‘Subhaa-nAllaah’ so as to alert him to wait for me or [even] say ‘Indeed. Allaah is with those who are patient’ so that he waits for me or should I not do any of the above?
Response: If the imaam goes into [the] rukoo’ [position] whilst you are inside the masjid, then calmly walk [towards the congregation and join them]. Whatever of the prayer you are able to pray with the congregation then do so and whatever you have missed, then make it up. It is not legislated for you to say anything to alert the imaam to wait for you from that which we know of the pure Sharee’ah.