Reference: Qa’idah Jaleeyaah: P. 75
Whoever says to someone else; ‘make Du’aa for me/us’ with the intention that the person being asked would benefit by making Du’aa, and that the person making the request would also benefit due to the action of the one being requested – just as he would benefit by requesting someone else to do any of the virtuous deeds, then this person is following the example of the Prophet – صلي الله عليه و سلم -, this is not considered to be from the questions that are not preferred.
As for the one whose only intention [by his request] is that he himself can attain his need, without intending to benefit the one whom he is making the request of, then this person is not amongst those who have taken the Messenger – صلي الله عليه و سلم – as an example in this issue. Rather this type of request is not preferred, forsaking it and turning to Allaah in need and asking Him is better than turning to the creation in need and asking them. And all of this is from the category of asking the living and is permissible and legislated.
Reference about this
 The Hadeeth collected by al Imaam Ahmad, at Tirmidhi and Abu Dawood that ‘Umar ibnul Khataab asked the Prophet for permission to make ‘Umrah, so he permitted him and said: ‘Do not forget us, my brother, in your Du’aa.’ Shaykh al Albaanee has declared the Hadeeth to be inauthentic in his notes to Mishkaat al Masaabeeh and Da’eef al Jaami’ as was clarified by Shaykh Rabee’ in his notes to this book.