Mus'ab b. Sa'd reported on the authority of his father. I was ailing. I sent message to Allah's Messenger (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 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.

وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا الْحَسَنُ بْنُ مُوسَى، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا سِمَاكُ، بْنُ حَرْبٍ حَدَّثَنِي مُصْعَبُ بْنُ سَعْدٍ، عَنْ أَبِيهِ، قَالَ مَرِضْتُ فَأَرْسَلْتُ إِلَى النَّبِيِّ صلى الله عليه وسلم فَقُلْتُ دَعْنِي أَقْسِمْ مَالِي حَيْثُ شِئْتُ فَأَبَى ‏.‏ قُلْتُ فَالنِّصْفُ فَأَبَى ‏.‏ قُلْتُ فَالثُّلُثُ قَالَ فَسَكَتَ بَعْدَ الثُّلُثِ ‏.‏ قَالَ فَكَانَ بَعْدُ الثُّلُثُ جَائِزًا ‏.‏