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.
If not the confusion created by Elie's running through the selection in Section 5, another instance where he saves his father from selection comes at the end of Section 6. Those who were weak were sent to the left while those who could walk well were sent to the right. Elie's father is sent to the left. Elie runs after his father (though he is strong enough to be sent to the right), and several of the SS officers follow after him. This creates enough confusion in the crowd that several men (including Elie and his father) who are going toward the left end up switching over to the right. Some others, on the other hand, are shot.