If none of them intended by the word “honestly” swearing by other than Allah, but just entrusting his Muslim brother with telling him the truth, there is nothing wrong with this at all. However, they should not have used this expression which in a literal sense means swearing by honesty. (Part No. 1; Page No. 343) If their intention was swearing by honesty, it is considered swearing by other than Allah, which is a form of Al-Shirk-ul-Asghar (minor form of associating others in worship with Allah), and one of the major sins.
`Umar ibn Al-Khattab (may Allah be pleased with him) narrated that the Messenger of Allah (peace be upon him) said,
Whoever swears by anything other than Allah has committed Kufr (disbelief) or Shirk (associating others with Allah in His Divinity or worship).
He (peace be upon him) also said,
Whoever swears by honesty is not one of us (i.e. the Muslims).
Ibn Mas‘ud (may Allah be pleased with him) said, “I would rather swear by Allah falsely than swear by other than Him in truth.”May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.