Reference: I'laam al-Mu'aasireen bi-Fataawa Ibn `Uthaymeen p55
Question: If a person [intending to perform the congregational prayer] finds the imaam in sujood, should he wait until he arises [from the sujood] or should he [immediately] join him [in sujood]?
Response: It is preferable for him to join the imaam in whatever position he finds him in and not to wait; this is based upon the generality of his ﷺ statement:
«And whatever you are able to catch (of the prayer) then pray...» [transmitted by al-Bukhaaree no.635 and Muslim no.603]
