Tariq bin Shihab (may Allah have mercy on him) said: Sayyiduna Umar (may Allah be pleased with him) took a shoulder blade (to write on) and gathered the Companions of Muhammad (peace and blessings be upon him) in order to write (a ruling on) the grandfather, and they thought that he would make him (equal to) a father. Then a snake came out at him and they dispersed. He said: "If Allah had wanted to carry it through, He would have carried it through."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12414