Abu Hurayra reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "When one of you wishes for something, he should look to what he desires. He does not know what he will be given."  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا تَمَنَّى أَحَدُكُمْ فَلْيَنْظُرْ مَا يَتَمَنَّى فَإِنَّهُ لاَ يَدْرِي مَا يُعْطَى