Because you say "this time" I assume you mean the second time he asks -- not the first one at the beginning of the play.
In that case, what you are looking for is found in Act IV, Scene 1. We do not actually see Oberon ask Titania for the child. Instead, we hear him telling Puck about what happened.
He tells Puck that Titania was quite happy to give him the child. As soon he asked, she said yes and sent a fairy to get the child and bring him to Oberon's home.
Oberon says she responded this way because he was making fun of her for being in love with donkey-headed Bottom.
A lovely boy stolen from an Indian king.She never had so sweet a changeling.And jealous Oberon would have the childKnight of his train, to trace the forests wild.But she perforce withholds the lovèd boy,Crowns him with flowers, and makes him all her joy. Act II, 1
Contagious fogs, which falling in the landHave every pelting river made so proudThat they have overborne their continents.The ox hath therefore stretched his yoke in vain,The ploughman lost his sweat, and the green cornHath rotted ere his youth attained a beard. Act II, 1
And for her sake do I rear up her boy,And for her sake I will not part with him. Act II, 1
When I had at my pleasure taunted herAnd she in mild terms begged my patience,I then did ask of her her changeling child,Which straight she gave me and her fairy sentTo bear him to my bower in Fairyland.And now I have the boy, I will undoThis hateful imperfection of her eyes. Act IV, 1