Al-Sha'bi (may Allah have mercy on him) said: Sayyiduna Ali and Sayyiduna Zayd (may Allah be pleased with them both) used to make one grandmother (paternal or maternal), or two, or three heirs to the sixth, neither reducing them below it nor increasing them above it, when all of them were equally near to the deceased. If one was nearer, the sixth was for her to the exclusion of the others. Sayyiduna Abdullah (may Allah be pleased with him) used to make the near and the distant ones all partners in the sixth, even if they were in different positions, and he regarded only the mother as a hajib (one who blocks inheritance) for the grandmothers.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12361