Reference: http://www.alfawzan.af.org.sa/node/16063
Question: Is it legislated for the imaam – whilst in rukoo’ – to wait for those inside the masjid to join him [and the congregation in prayer]?
Response: Yes it is. When the Messenger of Allaah (sal-Allaahu ‘alayhi wa sallam) had entered the rukoo’ [position], he would not raise his head [from this position] until he could no longer hear the sound of footsteps [approaching]; [so yes], he would wait for those who had [just] entered [the masjid].
