Praise be to Allah, and peace and blessings be upon the Messenger of Allah and his family.
What the questioner has mentioned about the report of Mujaahid was narrated by ad-Daarimi (may Allah have mercy on him) in his book an-Naqd (p. 463) via Sufyaan ibn ‘Uyaynah from Humayd al-A‘raj from Mujaahid.
In the commentary of Shaykh Mansoor as-Saamiri on an-Naqd, it says that this report was narrated by ‘Abdullah ibn Ahmad ibn Hanbal in his book as-Sunnah (hadeeth 1085, 1181) from Mujaahid from ‘Ubayd ibn ‘Umayr, without any mention of touching the knee, and he classed it as saheeh from him.
There are other narrations and versions of the report from Mujaahid, ‘Ubayd ibn ‘Umayr, Sa‘eed ibn Jubayr and others, with isnaads some of which are saheeh and some are da‘eef. All of these narrators are among the senior Taabi‘een. Even if these people were narrating from the Prophet (blessings and peace of Allah be upon him), these reports would be classed as mursal and da‘eef, but they did not narrate that from the Prophet (blessings and peace of Allah be upon him); perhaps they narrated it from the books of the Children of Israel.
Whatever the case, the basic principle of Ahl as-Sunnah (may Allah have mercy on them) was not to ascribe any attribute to Allah, may He be exalted, on the basis of the words of any of the Sahaabah or Taabi‘een; rather attributes can only be ascribed to Allah, may He be exalted, on the basis of the texts of the Quran and saheeh Sunnah. The attributes of Allah, may He be glorified and exalted, are tawqeefi, which means that nothing of that nature can be ascribed to Him except that which Allah ascribed to Himself or that His Messenger (blessings and peace of Allah be upon him ascribed to Him, because no one is more knowledgeable about Allah than He Himself, and no created being is more knowledgeable about his Creator than the Messenger of Allah (blessings and peace of Allah be upon him).
At the same time, Ahl as-Sunnah do not deny any attribute for Allah, may He be exalted, except that which Allah Himself negated or that His Messenger (blessings and peace of Allah be upon him) negate.
Seeing as the “knee” is not proven in any verse or saheeh hadeeth, we do not ascribe it to Allah, may He be exalted, because it is not proven. But we do not deny it either, because there is no text of the Quran or Sunnah to that effect. If this attribute was proven in the texts of the Quran or Sunnah, then our attitude towards it would be the same as our attitude towards all the other divine attributes that are proven in the Quran and Sunnah, which is to affirm it and to believe in it without any distortion, denial, likening Him to His creation or discussing how.
We have not seen this attribute affirmed in the books of Shaykh al-Islam Ibn Taymiyah or Ibn al-Qayyim (may Allah have mercy on them), and it has not been confirmed by our two contemporary shaykhs, Ibn Baz and al-‘Uthaymeen (may Allah have mercy on them).
And Allah knows best.
