The Messenger of Allah (peace and blessings be upon him) said:

“Settlement is permissible among Muslims, except a settlement that makes the unlawful lawful or the lawful unlawful. Muslims are bound by their conditions.”

In one report, Yahyā ibn Maʿīn said of Kathīr ibn Zayd, “He is trustworthy,” while in another he judged him weak.

Al-Tirmidhī and al-Bazzār narrated through Kathīr ibn ʿAbd Allāh ibn ʿAmr ibn ʿAwf al-Muzanī, from his father, from his grandfather, that the Messenger of Allah (peace and blessings be upon him) said:

“Settlement is permissible among Muslims, except a settlement that makes the lawful unlawful or the unlawful lawful. Muslims are bound by their conditions, except a condition that makes the lawful unlawful or the unlawful lawful.”

Al-Tirmidhī said, “This is a sound and authentic hadith.” Ibn Mājah narrated the first part of it. However, the hadith critics judged Kathīr ibn ʿAmr weak. Ahmad struck out his report in the Musnad and did not narrate it. Perhaps al-Tirmidhī graded it authentic because it was transmitted through several routes.

Abū Bakr al-Bazzār also narrated from Muhammad ibn ʿAbd al-Rahmān ibn al-Baylamānī, from his father, from Ibn ʿUmar, who said: The Messenger of Allah (peace and blessings be upon him) said:

“People are bound by their conditions insofar as those conditions accord with the truth.”

Although each of these chains of transmission is weak individually, their convergence through different routes allows them to strengthen one another. This meaning is supported by the Quran and the Sunnah, and it is the substance of this legal position. For a person laying down a condition has no right to permit what Allah has forbidden, nor to forbid what