Från: Umm_Amatillaah (Ursprungligt meddelande) Skickat: 2008-06-23 12:48
Reaching the Imaam while he is Bowing
Whoever joins the prayaner congregation while the Imaam is bowing, he must say the initial takbeer (i.e. Allaahu Akbar) whilst standing up with his back straight and being completely relaxed. Then he should make another takbeer to transition into the bowing position.
As for making takbeer when one has his back crooked and whilst already moving into the bowing position, this is not allowed at all. In fact, it may be a cause for his prayer to be nullified.
Imaam Ibn Qudaamah said in al-Mughnee (1/544): "A person that comes late to prayer must make the opening takbeer while his back erect. If he does it after having completely bent his back to go into the bowing position or whilst in the process of bending his back to bow, it is not valid, since he has done it in other than its proper place" 
The above is taken from:
Drawing Attention to some Mistakes made in Prayer
AUTHOR: Shaikh 'Alee Hasan Al-Halabee
SOURCE: Muntaqaa Fadaa'il as-Salat wa Adaabihaa (pg. 48-64)
PRODUCED BY: Al-Ibaanah.com