It is narrated from Sayyidah Umm al-Hasan (may Allah be pleased with her) that she saw the Mother of the Believers, Sayyidah Umm Salamah (may Allah be pleased with her), praying on a pillow because of an ailment in her eyes.
It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that he gave a concession regarding prostrating on a pillow and a cushion.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3675