Why is it impermissible to seek intercession by virtue of the Prophet's standing (peace be upon him) when supplicating, despite the existence of evidence indicating its permissibility, such as Umar's statement, "We seek intercession with You through the uncle of our Prophet," and Imam Ahmad's statement, "And ask God for your need through His Prophet (peace be upon him), and it will be fulfilled by God," and the absence of established disapproval of this by the early Muslims?

It is not permissible to seek intercession through the Prophet's ﷺ prestige (Jah) when supplicating, due to the lack of evidence for it. Umar’s seeking intercession through Al-Abbas, may Allah be pleased with him, is evidence for this. For if seeking intercession through the Prophet’s prestige were legislated, Umar would have done so. However, he sought intercession through Al-Abbas's supplication, not through his prestige.

Al-Zubair ibn Bakkar elucidated that Al-Abbas supplicated to Allah saying: "O Allah, no affliction descends save for a sin, and none is removed save through repentance. These people have turned to me because of my kinship to Your Prophet. Our hands are extended to You with sins, and our forelocks are inclined to You in repentance, so grant us rain."

As for what has been narrated from Imam Ahmad regarding the permissibility of seeking intercession through prestige, what is considered is that which is supported by evidence, and the prohibition is the more preponderant view.