Are all the hadiths found in "Kitab al-Du'a" in al-Musannaf authentic, or are some of them weak? And are common people who use it for supplication sinful due to the possibility of there being weak or fabricated hadiths within it?

The book "Al-Musannaf" by Ibn Abi Shaybah collected the hadiths of the Prophet, peace and blessings be upon him, and the narrations of the Companions and Tabi'un. Its author did not stipulate that it be restricted to only the authentic (sahih); rather, he compiled narrated reports as long as they were not fabricated or forged.

Therefore, every report within it requires a study of its chain of narration (isnad) and text (matn) to determine its acceptance or rejection. This applies to the book "Ad-Du'a" (Supplication) within it.

For those who wish to supplicate with what is authentic and well-established, they can refer to the chapters on supplication in Sahih al-Bukhari and Sahih Muslim, as Ibn Abi Shaybah was the teacher of the two Shaykhs (Bukhari and Muslim) and his hadiths are present in their works.

It is permissible for a Muslim to supplicate with whatever he wishes, even if it is not a reported tradition (ma'thur) or is not considered authentic according to the criteria of hadith scholars, provided there is no transgression in the supplication. It is recommended to observe the manners of supplication, such as humility and persistence, with a preference for adhering to prophetic wordings.