Technical note / Correction: The mention of "Ibn Abbas" has dropped out of manuscript (B), giving the false impression that it is a narration of Ibn Umar (i.e. directly from the Prophet (peace and blessings be upon him)), whereas Ibn Umar narrated it through "Ibn Abbas".

Grading: This hadith is "sahih li-ghayrihi", and this chain is "hasan" because it contains al-Mubarak bin Fadalah (Ibn Umayyah al-Basri).
Technical note / Defect ('illah): Al-Mubarak bin Fadalah explicitly stated his hearing from Ubayd Allah bin Umar in al-Tabarani's "al-Awsat" (579), so the suspicion of his tadlis is removed.

Reference / Source: Ibn Asakir narrated it in "Tarikh Dimashq" (44/441-442) via al-Hasan bin Muhammad al-Za'farani > Shababah bin Sawwar.
Details of the narration: This narration is in the account of when the Magian stabbed Umar, and Ibn Abbas went in to him and said: "May Allah reward you with good. The Messenger of Allah (peace and blessings be upon him) prayed that Allah would strengthen the religion through you while the Muslims were in hiding; then when you accepted Islam, the religion was strengthened through you." Ibn Asakir (28/44) also narrated similar wording via Nasr bin Hammad > al-Mubarak bin Fadalah.

Note: See the narration after it.

Reference / Source: Al-Tirmidhi (3683) narrated as "marfu'" via al-Nadr Abu Umar > Ikrimah > Ibn Abbas: "O Allah, strengthen Islam through Abu Jahl bin Hisham or through Umar bin al-Khattab."
Grading: Al-Tirmidhi said: "This hadith is gharib through this chain, and some scholars have criticised al-Nadr Abu Umar." We say: he is a "matruk and saqit" (abandoned and discarded, extremely weak) narrator.

Corroborating chains and supporting reports: The text of al-Mubarak bin Fadalah's hadith is supported by the hadith of A'ishah (may Allah be pleased with her), which comes later with the author.