Could you please inform me of the hadith? Also give the Arabic of the hadith too.
The numbering of hadiths was not done by Imam Ahmad ibn Hanbal himself; rather, it was done by publishers of his Musnad. This means that hadith 3400 could be a different one based on the publication.
I am posting the hadith based on the English Translation of Musnad Imam Ahmad Bin Hanbal, Vol. 3 (Hadiths 2823-4376), pp. 209, published by Darussalam in 2012, ISBN 978-603-500-107-6, which you can download from here:
٣٤٠٠ – حدثنا إسماعيل: أخبرنا أيوب عن عكرمة عن ابن عباس أن النبي ﷺ تزوج ميمونة وهو محرم
3400 . It was narrated from Ibn 'Abbas that the Messanger of Allah (ﷺ) married Maimoonah when he was in ihram.