Shaykh al-Islaam Ibn Taymiyyah (rahimahullaah) stated as occurs in Majmu' al-Fatawa (1/193):
ومن قال لغيره من الناس : ادع لي - أو لنا - وقصده أن ينتفع ذلك المأمور بالدعاء وينتفع هو أيضاً بأمره ويفعل ذلك المأمور به كما يأمره بسائر فعل الخير فهو مقتد بالنبي (صلى الله عليه وسلم) ، وأما إن لم يكن مقصوده إلا طلب حاجته لم يقصد نفع ذلك والإحسان إليه ، فهذا ليس من المقتدين بالرسول المؤتمين به في ذلك ، بل هذا من السـؤال المرجـوح الذي تـركه إلى الرغـبـة إلى الله ورسـوله أفضل من الرغبة إلى المخلوق وسؤاله
And whoever said to another (person) amongst the people: "Make du'a (supplication) for me" or "for us" - and his intent is to benefit that person commanded to make supplication and to benefit himself as well by commanding him, and that person commanded does (what he was commanded with), just as he (may) command him with all the (other) actions of goodness, then he (the one requesting this du'a in this manner, with this intent) is a follower of the Prophet (sallallaahu alayhi wasallam).
As for when his intent is but to seek his need and he did not desire to benefit that (person) and be benevolent towards him [by making him earn reward for performing worship through supplication] then this one is not from the followers of the Messenger, (from those) who mimic him in that regard. Rather, this is from the non-preferable (deeds) which if abandoned through aspiring for Allaah and (following his) Messenger is superior to aspiring for the creation and asking them.
This is another manifestation of the perfection in one's Tawhid when a person follows the way of the Messenger (sallallaahu alayhi wasallam) which is that the Messenger (sallallaahu alayhi wasallam), when he asked others to supplicate, his intent was to benefit the person whom he asked, by commanding him with a righteous deed through which he will be rewarded, so this is ihsaan (benevolence) to that other person. So whoever follows this way and makes sure that his requests for supplication from others are not purely just to benefit himself, then he is a follower of the Messenger (sallallaahu alayhi wasallam). And through this, a higher rank in the perfection of one's Tawhid is attained.
These are understandings and implmentations of Tawhid and its perfection that are relished by the Muwahhidoon. Such delights and pleasures which the saint and grave worshippers are deprived of, those who humiliate and lower themselves by aspiring to other than Allaah and making known their abject need from them, in both speech and deed. So when the difference in these two intents behind asking others to supplicate for you indicates a variation in the perfection of Tawhid and in making ittibaa' (following) of the Messenger, then what great difference is there in rank, honour and dignity between the followers and imitators of the Messenger (sallallaahu alayhi wasallam) in the perfection of Tawhid and those besides them from those whose aspirations for their dead saints reach the level of Major Shirk?!