To what extent is the hadith sound that Ibn Kathir narrates in his tafsir of verse 24 of Surat Ibrahim, which refers to the virtue of dhikr after prayer, and does a Muslim receive reward if he acts upon a weak hadith before knowing of its weakness?

The hadith narrated about the virtue of glorifying, praising, declaring God's oneness, and exalting God after prayers was reported by Ibn Abi Hatim, and its narrators are trustworthy, but its apparent defect is irsal (a mursal hadith); because Qatadah is one of the younger Tabi'in and his hearing from the Companions has not been authentically established except from Anas ibn Malik, may God be pleased with him. The mursal narrations of Qatadah are not accepted by scholars, and many researchers consider them mu'dal (perplexing) or munqati' (disconnected) because he usually omits two narrators from the chain of transmission.

Therefore, the hadith is weak and not authentic due to irsal.

Despite its weakness, if a weak hadith is narrated concerning the virtues of deeds, and the origin of the act of worship is sanctioned by authentic evidence, then it is permissible for a Muslim to act upon it hoping for reward, as long as the worship is not restricted by a characteristic or form not found in the Sharia.

Glorifying, praising, declaring God's oneness, and exalting God after prayers is sanctioned by authentic hadiths, and there is no harm in performing them—at times—in the sequence mentioned in the weak hadith, with the hope of the reward mentioned therein. It is something that will not cause harm if it does not benefit, even though the well-known formula (33 tasbihs, 33 tahmids, 33 takbirs) is better.