Can I donate all my money to charity and not leave a single cent for my heirs? What is the basis of the condition of Wasiat, mainly on the maximum of one-third of total estate - that is allowed to be willed away to non-Faraid beneficiaries?

It is recorded in the Sahihain that Sa`d bin Abu Waqqas said, "O Allah's Messenger ﷺ! I have some money and only a daughter inherits from me, should I will away all my remaining property (to others)"

He ﷺ said, "No." Sa`d said, "Then may I will half of it" He ﷺ said, "No."

Sa`d said, "One-third" He ﷺ said, "Yes, one-third, yet even one-third is too much. It is better for you to leave your inheritors wealthy than to leave them poor, begging from others."

Ibn `Abbas said, I recommend that people reduce the proportion of what they bequeath by will to a fourth (of the whole legacy) rather than a third, for Allah's Messenger ﷺ said:

«الثُّلُثُ وَالثُّلُثُ كَثِير»

"One-third, yet even one-third is a lot."

(Sahih al-Bukhari)