Muwatta Imam Malik Book of Wills and Testaments, Hadith 1

Malik related to me from Nafi from Abdullah ibn Umar that the Messenger of Allah ﷺ said, "It is the duty of a muslim man who has something to be given as a bequest not to spend two nights without writing a will about it."
Malik said, "The generally agreed-on way of doing things in our community is that when the testator writes something in health or illness as a bequest, and it has freeing slaves or things other than that in it, he can alter it in any way he chooses, until he is on his deathbed. If he prefers to abandon a bequest or change it, he can do so unless he has made a slave mudabbar (to be freed after his death). If he has made him mudabbar, there is no way to change what he has made mudabbar. He is allowed to change his testament because the Messenger of Allah ﷺ said, "It is the duty of a muslim man who has something to be given as a bequest not to spend two nights without writing a will about it."
Malik explained, "Had the testator not been able to change his will nor what was mentioned in it about freeing slaves, each testator might withhold making bequests from his property, whether in freeing slaves or other than it. A man gives a bequest in his health and in his travelling." (i.e. he does not wait till his death bed ) .
Malik summed up, "The way of doing things in our community about which there is no dispute is that he can change whatever he likes of that except for the mudabbar."

حَدَّثَنِي مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ " مَا حَقُّ امْرِئٍ مُسْلِمٍ لَهُ شَىْءٌ يُوصَى فِيهِ يَبِيتُ لَيْلَتَيْنِ إِلاَّ وَوَصِيَّتُهُ عِنْدَهُ مَكْتُوبَةٌ " .