It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) regarding the saying of Allah Almighty: "And let those fear who, if they left behind" [Surah al-Nisa: 9]: that is, a man would be present with another at the time of his death and hear him making a bequest that harms his heirs. So Allah Almighty ordered the one who hears him to fear Allah, to make him understand and guide him to what is right, and to look after his heirs as he would like his own heirs to be treated if he feared destitution for them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12582