As Salaamu Alaikum.While praying Isha prayer I was leading my father in prayer when I raised my head from sujood and sat before saying takbeer my father also raised his head lit bit from sujood not fully and he went back to sujood again .so is My father's prayer valid?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

What you mentioned about your father raising his head from the prostration before hearing the Takbeer from you does not affect the validity of his prayer. Some scholars have stipulated that if the follower hastens and proceeds to an action after the Imam has finished with the obligatory acts of prayer, then there is nothing wrong with that as Al-Bahooti mentioned in Kashshaf al-Qinaa’ that there is no harm on the follower (praying behind the Imam) to precede the Imam or to do the action at the same time like him in any of the words of the prayer as long as that is not in the Takbeeratul-Ihraam; as regards other words (of the prayer), then it is only recommended to agree with him.” [End of quote]

Al-Bahooti also said in Muntaha Al-Iraadaat: “It is more appropriate for him (i.e. for the person led in prayer) to initiate the actions of the prayer after the Imam, and if he agrees with him [does the actions at the same time like him], then this is disliked.” [End of quote]

Therefore, your father was not obligated to return to the prostration because of what we have mentioned about the permissibility of the follower following the Imam in an action before hearing the Takbeer; perhaps he did that thinking that he had to go back to the prostration, but he had only to do that if he had raised his head before the Imam had had raised his own; in any case, his prayer is valid.

It should be noted that according to the preponderant opinion, it is desirable for the Imam to begin the Takbeer at the beginning of his raising from the prostration, and not after sitting straight. Ibn Qudaamah said in Al-Mughni: “It is recommended for the praying person to start saying the Takbeer when starting to raise his head from the Sujood (prostration), and finishing saying the Takbeer when he sits up straight.” [End of quote]

The matter is easy in this regard, Allah willing.

Allah knows best.