Is the prayer invalid if we do not keep our back straight?

The straightening of the back in prayer occurs in three positions: 1. Standing (Qiyam): The Hanafis and Hanbalis held that the limit of standing is that one's hands should not reach the knees if extended. As for the Malikis and Shafi'is, they held that the prayer becomes invalid if the worshipper bends and is closer to bowing (Ruku') than to standing (Qiyam). In another opinion, it does not become invalid unless it reaches the limit of bowing. 2. Straightening up from Ruku' and Sujud: It is obligatory to straighten the back when rising from Ruku' and Sujud, based on the Hadith: "The prayer of a person does not suffice until he straightens his back in Ruku' and Sujud." According to Shafi'i, Ahmad, and Ishaq, the prayer becomes invalid if the worshipper does not straighten his back in Ruku' and Sujud. Ibn Taymiyyah said that straightening the back occurs when lifting the head from Ruku' and Sujud, and that this is part of their completion. Hudhayfah saw a man who did not complete his Ruku' or Sujud, and he said to him: "You have not prayed. And if you were to die, you would die upon other than the Sunnah of Muhammad (peace be upon him)."

It is also important that the worshipper does not bend in sitting (between the two prostrations or during Tashahhud) such that his forehead goes past his knees, lest his prayer becomes invalid.