Shaikh Al-Albaanee (rahimahullaah) discusses the Sunnah in observing the rukoo’ behind the Imaam in a congregational prayer; and when those behind the Imaam should proceed to go into sujood after arising from the rukoo’ once the Imaam has already done so.
The Shaikh (rahimahullaah) stated before citing the hadeeth:
An obligation which is abandoned and unknown!!
((They would pray with the Messenger of Allaah (Sallallaahu ‘alaihi wa sallam), so if he would bow (in rukoo’) then they would bow, and when he said “sami’ Allaahu liman hamidah” (Allaah listens and responds to the one who praises Him) they would not cease to remain standing until they saw him having placed his face (and in a narration: his forehead) upon the ground. Then they would follow him.))
(Reported by Muslim (2/46 no. 1064), Aboo Dawood (no. 622) and from him Aboo ‘Awaanah (2/179) and others.)
The majority of the worshippers leave alone what should necessitate the delaying of the prostration until the Imaam has placed his forehead upon the ground. I cannot exempt anyone from them; even those of them that are ardent upon following the Sunnah due to ignorance of it or negligence regarding it – except whoever Allaah wills – and a few they are.
An-Nawawee (rahimahullaah) said in Sharh Muslim:
“In this hadeeth is a manner from the mannerisms of prayer, and it is that the Sunnah is that the followers (in prayer) do not bend down to prostration until the Imaam has placed his forehead upon the ground. Except in the case where it is known from his (the Imaam’s) routine that if he were to delay to this extent then the Imaam could arise from the prostration before his own prostration. Some of our companions (rahimahumullaahu ta’aalaa) have said: In this hadeeth as well as in other than it is what comprises its combination; which is that the Sunnah for the follower is to delay from the Imaam a little so that he commences in the pillar (of prayer) after its beginning – and before his completion of it.”
(Taken from: Silsilatul Ahaadeeth As-Saheehah 6/1/225-226)