Sheikh `Abd al-Rahmân al-Barrâk
Tawassul in supplication takes many forms. Some are permitted and some are not.
Permitted forms of tawassul are the following:
1. Calling on Allah by His names and attributes, like saying: “O Allah! I ask of you by your mercy…”
2. Calling on Allah and mentioning your good deeds. This was done by the three men trapped in the cave. Each one called upon Allah and mentioned something good that he had done. One had showed kindness to his parents. Another had abstained from a forbidden relationship. The third had shown generosity to another who had left his wealth with him.
As for tawassul by asking others to make supplications on your behalf, it is disliked. Some scholars have declared it forbidden, saying that the Prophet (peace be upon him) has forbidden us from asking people from begging, and this is a kind of begging.
Personally, I see nothing wrong with asking others to supplicate to Allah on your behalf, if you believe their supplications are more likely to be answered. As for making supplications on behalf of others, this is clearly permitted. Companions asked each other to make supplications on their behalf.
Other forms of tawassul are forbidden. It is forbidden to invoke other people in your prayers, like saying: “O Allah! I ask of you in the name of so-and –so.”
It is not permissible to seek supplications from those who have died or from those who are not present. The Companions stopped asking the Prophet (peace be upon him) to make supplications on their behalf after the Prophet (peace be upon him) had died. In the year of famine, `Umar said the following: “O, Lord, we were soliciting you by our Prophet and you would answer us and let us have rain, now we are soliciting you by the uncle of our Prophet so we ask you to answer us.” Then he asked the Prophet’s uncle to supplicate on their behalf.
The Companions clearly made a distinction between the living and the dead in this matter.
Source: Islam Today