Where is the part in "Night" where Wiesel creates chaos to save his father from selection?

It is most likely that you are speaking about the
selection in section 5, when Elie and his father are not on the same block for the
selection.  They have been warned that this selection is to decide between those who are
healthy enough to continue working and those who are not and advised to make themselves
look as healthy and strong as possible.  In an earlier translation, Elie and his father
are shown to approach Dr. Mengele together.  In his fear of having his number taken down
as one who is unfit, Elie runs through telling himself all the while that this will make
him seem strong.  When he arrives with others who have passed before him they tell him
he ran too fast to have his number written down.  In the commotion, Elie is surprised to
see that it seems his father has also been passed on and his number is not written
down.