All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, Tawassul means seeking Waseelah (a means) that leads to the aimed target. If by seeking Tawassul to Allaah by virtue of the Prophet you mean to seek Tawassul with his love, obedience and believing in him, then this is a valid Tawassul according to the agreement of the scholars because it is making Tawassul to Allaah through one’s good deeds. However, if you mean seeking supplication from him (the Prophet ) after his death, then this is an innovated Tawassul but it does not reach the degree of Shirk [associating partners with Allaah].
However, it is an act of foolishness to ask a dead person to supplicate Allaah on our behalf, because if a dead person dies, his actions come to an end and it is not possible for a dead person to supplicate (after his death) for someone else, even the Prophet does not supplicate to someone else after his death. It is for this reason that the Companions did not supplicate to Allaah by virtue of the Prophet after his death.
When the people were severely affected by drought in the era of ‘Umar the latter supplicated: “O Allaah, we used to request our Prophet to supplicate to You for rain and You would grant us rain, and now we are requesting the uncle of our Prophet (i.e. Al-Abbaas Ibn Abdul Muttalib) to supplicate to You for rain, so grant us rain." [Al-Bukhari] Then, Al-‘Abbaas stood and supplicated to Allaah. Had asking supplication from a dead person been permissible and a correct means, then ‘Umar, and the Companions who were with him would have asked this from the Prophet because his supplication is more likely to be accepted than the supplication of Al-‘Abbaas . Hence, it should be mentioned that Tawassul to Allaah by asking a dead person to supplicate Allaah on behalf of someone is invalid and it is not lawful or permissible.
If you mean to seek Tawassul with the high rank of the Prophet and his status with Allaah, then this Tawassul is an innovation as well and it has no Sharee’ah evidence as “the high rank of the Prophet is not beneficial for a supplicator because it is not beneficial except for the Prophet and not to anyone else, so that he will make Tawassul to Allaah through it.” [From Fataawa about the Pillars of Islam by Ibn ‘Uthaymeen]
Therefore, this kind of Tawassul “is forbidden and it is an innovation and the Companions were not used to do so.” [From Fataawa Noor ‘Al Ad-Darb of Ibn ‘Uthaymeen] For more benefit, please refer to Fataawa 89352 and 84006.
On the other hand, it is not acceptable to use the story of Moosa (Moses) on the night of M’iraaj (Ascension through the spheres of the heaven) to support that innovation (innovative Tawassul), because the Prophet did not make Tawassul through Moosa nor did he seek his intercession, but it was Moosa who took the initiative to advise the Prophet, sallallaahu ‘alayhi wa sallam, to go back to Allaah and ask for the number of the prayers to be reduced because Moosa had experienced the people (Children of ‘Isra’el) and he knew them, so he asked the Prophet to ask his Lord to reduce the number of the prayers and there was no Tawassul in this regard.
Moreover, the Companions did not understand from this story that it was permissible to make Tawassul through the dead people and had they understood this meaning, they would have sought the Tawassul of the Prophet after his death, but they did not do so as we mentioned in the story of Al-‘Abbaas .
Furthermore, the Prophet clarified to us the means to achieve his intercession on the Day of Judgment, and that is by achieving Tawheed (monotheism) as in the Hadeeth: “The happiest person who will have my intercession on the Day of Resurrection will be the one who said "None has the right to be worshipped but Allaah" sincerely from the bottom of his heart.” [Al-Bukhari]
Also, the Prophet’s intercession could be achieved by supplicating Allaah after hearing the ‘Athaan (call to the prayer) to grant him Al-Waseelah (which is the highest position in Paradise), as in the Hadeeth: “…then ask Allaah, to grant me Al-Waseelah which is a level in Paradise which only one of the slaves of Allaah will reach, and I hope I will be the one, so whoever asks Al-Waseelah for me, my intercession will be granted for him (on the Day of Resurrection)." [Muslim]
Therefore, the above are the means of achieving the intercession of the Prophet as he [Prophet] did not order us to ask him to supplicate for us after his death, and his Companions did not do so despite the fact that they were more keen to achieve good than us and they understood from Allaah and His Prophet more than us. For more benefit, please refer to Fataawa 14955, 16690 and 91551.
Allaah Knows best.