As regard those who bequeath that the Faatihah be recited on behalf of the Prophet’s soul, or on his behalf by the Prophet’s grave?
The Answer by Shaykh Uthaymeen (rahimahullaah):
It is not binding to execute this bequest, because it is a bequest of an unprescribed matter. For the Prophet (Sallallaahu alaihi wa sallam) does not prescribe for anyone to worship Allaah and then dedicate the reward of worship to the Messenger (Sallallaahu alaihi wa sallam). Since, had this been prescribed, then the companions, may Allaah be pleased with them, would have been the most preceding people in attaining it.
And also because the Prophet (Sallallaahu alaihi wa sallam) is in no need for this. For indeed there is no person who does a righteous deed except that the Prophet (Sallallaahu alaihi wa sallam) would have a similar reward. Because he (Sallallaahu alaihi wa sallam) is the one who has directed to this, and, [as the Prophet (Sallallaahu alaihi wa sallam) said]: “The one who guides to what is good is like the one who does it.”  So, this bequest is from the foolish and futile actions as well as a bid’ah, something which has not been reported from the righteous predecessors, may Allaah be pleased with them.
And similarly if he said: “Recite the Faatihah on my behalf by the Prophet’s grave,” then it is not binding to fulfill this bequest. The reason for this is that specifying a particular place for a particular worship not prescribed by the sharia, is from the bida’ as it is known in the aforementioned discussion pertaining to the issue of “Pursuance of the Messenger (Sallallaahu alaihi wa sallam).”  And that the following of the Messenger (Sallallaahu alaihi wa sallam) cannot be achieved until the act of worship agrees with the Sharee’ah in six matters: cause, type, extent, manner, time, and place.
 Reported by at-Tirmithee (no.2672, Arabic). And also in saheeh Muslim: “One who guides to something good has a reward similar to that of its doer”, vol.3, no.4665.
 See page section on “Saved Group”.
Translated by Dr Saleh as Saleh rahimahullaah
Source : Understanding Worship – Fiqh ul-‘Ibadah – Shaykh ibn Uthaymeen – Page 174