There is no specific scene in the book that depicts Judge Taylor directly asking Atticus to defend Tom Robinson. However, in chapter 9, Atticus describes how Judge Taylor asked him to take the case. Atticus is discussing the upcoming trial with his brother and says,

I really can’t tell at this stage, Jack. You know, I’d hoped to get through life without a case of this kind, but John Taylor pointed at me and said, "You’re It" (Lee, 91).

Leading up to the trial, both Jem and Scout are under the impression that their father made the independent decision to defend Tom Robinson on his own. In chapter 16, the children enter the crowded Maycomb courthouse and attempt to look for a good seat to watch the trial. Unfortunately, Scout gets stuck by the back staircase, where she overhears a group of old men and courthouse critics, known as the Idlers’ Club, discussing the trial. Scout is astonished when she overhears one of the men say,

Lemme tell you somethin' now, Billy . . . you know the court appointed him to defend this nigger (Lee, 165).

After Scout overhears the man's comment about Judge Taylor making Atticus take the case, she finds it strange that her father never said anything about it.

This question can be deceiving because there is a specific scene in the movie that shows Judge Taylor talking to Atticus on his front porch about taking the Tom Robinson case, but this does not explicitly happen in the book. The first time the reader knows anything about the case is in chapter nine when Scout tells Cecil Jacobs to take back his comment about Atticus defending "n*****s". Scout goes home and asks Atticus if it is true and he tells her about Tom Robinson and the case that will take place the following summer. 

The only other reference in the book about Judge Taylor asking Atticus to be Tom Robinson's attorney is during Christmas time. Scout overhears her father talking to Uncle Jack about it, as follows:

"You know, I'd hope to get through life without a case of this kind, but John Taylor pointed at me and said, 'You're It'" (88).