Bayhaqi 18473 — Permission to recite battle verse (rajaz) during

Ibn Ishaq mentions the account of the martyrdom of Sayyiduna Ja'far bin Abi Talib (may Allah be pleased with him) in the Battle of Mu'tah, and says (that he was reciting):
O Paradise! You have drawn near; your drink is pure and cool.
And the Romans are a people whose punishment has drawn near; if I meet them, killing them is my duty.
Abdullah bin Abi Bakr bin Hazm said: When Sayyiduna Abdullah bin Rawahah (may Allah be pleased with him) took the banner that day, he recited these verses:
O soul! I have sworn that you shall descend into battle, whether gladly or unwillingly,
Even if the people gather and raise a fierce clamour. I see that you are averse to Paradise.
You have lived your life in ease; you are nothing but a drop of fluid.
Ibn Ishaq said: He also said:
O soul! If you are not killed, you will die anyway. This is the pool of death which you have reached, and what you wished for has been given to you. If you do what these two did (that is, follow Sayyiduna Ja'far and Sayyiduna Zayd (may Allah be pleased with them both)), it is better; and if you hold back, you are wretched.
He meant Sayyiduna Ja'far and Sayyiduna Zayd (may Allah be pleased with them both). Then he advanced and fought, and was martyred.