It is narrated from Umm al-Mu'mineen Sayyida Maymunah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) used to pray on a khumrah (small mat).
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1411
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده صحيح وسليمان هو: ابن أبي سليمان الشيباني. والحديث متفق عليه، [مكتبه الشامله نمبر: 1413]
(«خمرة») refers to that small piece upon which only the forehead can be placed for prostration (sajdah), whether that piece is made of mat, cloth, or is made from bark, grass, etc.