Sahih Muslim, 4212

Rapporté par Mus'ab b. Sa'd, d’après son père رضي الله عنه : J’étais malade. J’ai envoyé un message au Messager d’Allah ﷺ pour lui demander la permission de donner mes biens comme je le souhaitais. Il a refusé. J’ai demandé : « La moitié ? » Il a encore refusé. J’ai dit : « Alors un tiers ? » Le Prophète ﷺ est resté silencieux après que j’ai demandé la permission de donner un tiers. C’est alors que le don d’un tiers est devenu permis

Mus'ab b. Sa'd reported on the authority of his father. I was ailing. I sent message to Allah's Apostle (ﷺ) saying:Permit me to give away my property as I like. He refused. I (again) said: (Permit me) to give away half. He (again refused). I (again said): Then one-third. He (the Holy Prophet) observed silence after (I had asked permission to give away) one-third. He (the narrater) said: It was then that endowment of one-third became permissible