Mishkat al-Masabih 3072
Sa’d b. Abu Waqqas said:
God’s Messenger visited me when I was ill and asked whether I had made my will. I replied that I had. He asked how much I had willed and when I told him I had willed all my property away to be devoted to God’s path he asked how much I had left my children. I replied that they were rich and prosperous, whereupon he told me to will away a tenth ; but I kept on telling him it was too little till he finally said, “Will away a third, but a third is a lot.” 

Tirmidhi transmitted it.
عَن سَعْدِ بْنِ أَبِي وَقَّاصٍ قَالَ: عَادَنِي رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَأَنَا مَرِيضٌ فَقَالَ: «أَوْصَيْتَ؟» قُلْتُ: نَعَمْ قَالَ: «بِكَمْ؟» قُلْتُ: بِمَالِي كُلِّهِ فِي سَبِيلِ اللَّهِ. قَالَ: «فَمَا تَرَكْتَ لِوَلَدِكَ؟» قُلْتُ: هُمْ أَغْنِيَاءُ بِخَيْرٍ. فَقَالَ: «أوص بالعشر» فَمَا زَالَت أُنَاقِصُهُ حَتَّى قَالَ: «أَوْصِ بِالثُّلُثِ وَالثُّلُثُ كَثِيرٌ» . رَوَاهُ التِّرْمِذِيّ