This is an answer given by the Shaykh to the issue of requesting the supplication of those who have passed away and entered the barzakh. The Shaykh was asked (during his explanation of Aqidah al-Tahaawiyyah), in the print version 2/1029 (Dar al-Mawaddah, Egypt):

من سأل النبي صلى الله عليه وسلم أن يدعو له وأن يطلب له المغفرة من الله بعد موته، هل هذا شرك؟

The one who asked the Prophet (sallallaahu alayhi wasallam) to supplicate for him and to ask for his forgiveness from Allaah after his (the Prophet's) death, is this shirk?

The Shaykh's answer:

نعم، هو شرك أكبر لأن النبي صلى الله عليه وسلم لا يُدعى بعد موته، فطلب الدعاء من الميت، وطلب الدعاء بالإغاثة أو الاستسقاء؛ يعني أن يدعو الله أن يغيث، أو أن يدعو الله أن يغفر، أن يدعو الله أن يعطي ونحو ذلك، هذا كله داخل في لفظ الدعاء والله - عز وجل - قال {وَأَنَّ الْمَسَاجِدَ لِلَّهِ فَلَا تَدْعُوا مَعَ اللَّهِ أَحَدًا} [الجن:18] ، والذي يقول إنّ هذه الصورة وهي طلب الدعاء تخرج عن الطلب الذي به يكون الشرك شركًا فإنه ينقض أصل التوحيد كله في هذا الباب، فكل أنواع الطلب؛ طلب الدعاء يعني طلب الدعاء من الميت، طلب المغفرة من الميت، أو طلب الدعاء من الميت أن يدعو الله أن يغفر، أو طلب الإغاثة من الميت أو طلب الإعانة أو نحو ذلك كلها باب واحد هي طلب، والطلب دعاء فداخلة في قوله تعالى {وَمَنْ يَدْعُ مَعَ اللَّهِ إِلَهًا آخَرَ لَا بُرْهَانَ لَهُ بِهِ فَإِنَّمَا حِسَابُهُ عِنْدَ رَبِّهِ إِنَّهُ لَا يُفْلِحُ الْكَافِرُونَ} [المؤمنون:117] ، وفي قوله {وَأَنَّ الْمَسَاجِدَ لِلَّهِ فَلَا تَدْعُوا مَعَ اللَّهِ أَحَدًا} ، وفي قوله {وَالَّذِينَ تَدْعُونَ مِنْ دُونِهِ مَا يَمْلِكُونَ مِنْ قِطْمِيرٍ} [فاطر:13] ، ونحو ذلك من الآيات، فالتفريق مضاد للدليل، ومن فهم من كلام بعض أئمتنا التفريق أو أن هذا طلب الدعاء من الميت أنه بدعة لا يعني أنه ليس بشرك بل هو بدعة شركية, يعني ما كان أهل الجاهلية يفعلونه، وإنما كانوا يتقربون ليدعوا لهم، لكن أن يُطْلَبَ من الميت الدعاء هذا بدعة ما كانت أصلًا موجودة لا عند الجاهليين ولا عند المسلمين فحدثت فهي بدعة ولاشك، ولكنها بدعة شركية كفرية وهي معنى الشفاعة، إيش معنى الشفاعة التي من طلبها من غير الله فقد أشرك؟ الشفاعة طلب الدعاء، طلب الدعاء من الميت هو الشفاعة.

Translation:

Yes, it is major shirk because the Prophet (sallallaahu alayhi wasallam) is not to be called upon after his death. Requesting supplication from the dead and requesting (their) supplication (for them to) request rescue and sending of rain, meaning, that he supplicates to Allaah that He grants relief or he supplicates to Allaah to forgive (the person making the request), that he supplicates to Allaah to give (to that person) and what is like this, all of it is enters into the word "du'a" (supplication) and Allaah, the Mighty and Majestic said, "And the mosques are for Allaah, so do not invoke anyone alongside Allaah" (72:18).

And the one who says that this form, which is to request the supplication (of the deceased) is excluded from that (type of) request through which shirk is actually shirk, then he has demolished the entire foundation of Tawheed in this particular topic.

For all types of request (talab), requesting supplication, meaning requesting supplication from the dead or requesting aid and what is like that, all of it is included within the same subject area, which is (making) request (talab), and requesting is supplicating (du'a), so it is included within His, the Most High's saying, "And whoever invokes besides Allah, any other ilah (god) of whom he has no proof then his reckoning is only with his Lord. Surely, the disbelievers will not be successful." (23:117) and it is also included within His saying, "And the mosques are for Allaah, so do not invoke anyone alongside Allaah" (72:18) and into His saying "And those, whom you invoke or call upon instead of Him, own not even the thin membrane over the dateŭstone" (35:13) and other similar verses.

Hence, making this distinction clashes with the evidence and whoever understood from some of the speech of our Scholars (this) separation or that this requesting of supplication from the dead is a bid'ah (innovation), this does not mean it is not (at the same time) shirk, rather it is an innovated (form) of shirk, with the meaning that the people of Jaahiliyyah did not used to perform this (particular type of shirk). They used to seek nearness to (their deities) so they in turn would supplicate for them. However, that the dead are requested to supplicate, this is a bid'ah, it was not present originally, neither with the people of Jaahiliyyah and nor with the Muslims, then it occurred, and so it is an innovation no doubt (from this angle), however it is an innovation of shirk and kufr. And this is the (very) meaning of shafaa'ah (intercession), what is the meaning of intercession, that (intercession) which if a person seeks from other than Allaah he becomes a mushrik? intercession (means) to request supplication, requesting supplication from the dead, this is (the meaning) of intercession.