Many times when a Muslim does a favor for another Muslim the one receiving the favor will thank them, so they will reply by saying, "do not thank me just make du’aa for me." The Scholars have explained that this is not correct.
The following is a summary translation taken from the website of Shaikh Saalih al-Fawzan (hafidhahullaah):
Shaikh Fawzan said, (in his book Highlights of the religious verdicts of Shaikh-ul Islam Ibn Taymiyaah in the creed) that Shaikh-ul Islam Ibn Taymiyaah (rahimahullaah) said:
And from the manners of the one who gives to others or spends upon others is that they should not seek a favor based on what they have given, nor should they seek a reward from the creation based upon what they have given. And from the rewards is to request supplication (du’aa).
Allah says about those He has commended: we only feed you seeking the Face of Allaah; we do not want from you any reward or any thanks. (Al-Insan, 76:9)
And supplication is a reward, as comes in the Hadith, whoever does a favor for you then return the favor to them; and if you are not able to find anything to return the favor with, then supplicate for them until you know that your supplication has equaled their favor.
And ‘A`isha (radiallaahu ‘anha), when she would send out charity, she would say to the one delivering the charity for her, "listen to the supplication that they make for us, so that you can make the same supplication for them that they made for us and therefore our reward can remain with Allaah."
Some of the pious predecessors would say, if a beggar says to you: "may Allah bless you," then say to them, "and may Allah bless you as well."
Therefore, whoever does well to the creation, whether they are from the Prophets, or righteous people, or a king or a rich person; then this good must be done sincerely for the Face of Allaah – only seeking the Face of Allaah. And they should not request from the creation any reward, or supplication or anything else; not from a Prophet nor from a righteous man because Allaah has commanded His slaves – all of them – to worship Him making the worship sincerely for Him alone.