The Book of Oaths (Kitab Al-Aiman)
Muslim :: Book 15 : Hadith 4056
Tamim b. Tarafa reported: A beggar came to 'Adi b. Hatim and he begged him to give him the price of a slave, or some portion of the price of the slave. He ('Adi) said: I have nothing to give you except my coat-of-mail and helmet. I will, however, write to my family to give that to you, but he did not agree to that. Thereupon 'Adi was enraged, and said: By Allah, I will not give you anything. The person (then) agreed to accept that, whereupon he said: By Allah, had I not heard Allah's Messenger (may peace be upon him) saying:" He who took an oath, but then found something more pious in the sight of Allah, he should (break the oath) and do that which is more pious," I would not have broken the oath (and thus paid you anything).
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