Musnad Abu Ya'la al-Mawsili 6:91

Abu Khaythama narrated to us, Jarir narrated to us, from ‘Ata’ ibn al-Sa’ib, from Abu ‘Abd al-Rahman, from Sa’d ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) visited me while I was ill and said: "Have you made a will?" I said: "Yes." He said: "For how much?" I said: "For all my wealth in the way of Allah." He said: "Then what have you left for your children?" I said: "They are wealthy and well-off." He said: "Make a will for a tenth." I continued to bargain with him, and he bargained with me, until he said: "Make a will for a third, and a third is much." Abu ‘Abd al-Rahman said: We consider it desirable to bequeath less than a third, due to the saying of the Messenger of Allah (may Allah bless him and grant him peace): "And a third is much."