Asalam alaykum. I was prayin with the imam. When the imam went on rukuu, i did not do that on time until he raised up. So, tel me am i going to pay back that rakaat back or no need?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If your being delayed in going into Rukoo’ (bowing down) behind the Imaam was due to a sound reason such as for the purpose of completing the recitation of Chapter Al-Faatihah, then you should perform Rukoo’ and then rise up from Rukoo’ and then catch up with the Imaam; you are not required to do anything else.

However, if your being delayed (in going into Rukoo’ with the Imaam) was without a sound reason, then some scholars are of the view that whoever is preceded by the Imaam with one pillar without a sound reason, then his prayer becomes void unless he does not know the ruling.

“Mataalib 'Uli An-Nuha”, one of the Hanbali books, reads: “If a person led in prayer is preceded by the Imaam with one pillar which is related to the acts of the prayer (like standing, Rukoo’, Sujood, etc) whatsoever or more than one pillar without a sound reason, then the prayer of someone who did so deliberately becomes void, while the prayer of someone who did so out of ignorance or forgetfulness is valid.”

However, according to the Shaafi'i School of jurisprudence, the prayer is valid as long as one is preceded by the Imaam only with one pillar whether or not this was due to a sound reason. The Fiqh Encyclopedia reads: “The Shaafi'i School of jurisprudence stated that the person led in prayer is obliged to follow the Imamm in the acts of the prayer and if he is preceded by the Imaam with one pillar, then his prayer does not become void.” For more benefit, please refer to Fatwa 129153.

Therefore, if you did not know the ruling and you was delayed in following the Imaam until he raised his head from Rukoo’, then you should perform Rukoo’ and then rise up from it [Rukoo’] and then catch up with the Imaam; you are not required to do anything else. Also, you do not have to make up for this Rak’ah after the Imaam concludes the prayer with Salaam.

Allaah Knows best.