Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) reports this as a "marfu'" hadith from the Prophet (peace and blessings be upon him), that he (peace and blessings be upon him) said: "Whoever uses stones (for cleansing) should use three."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Qays bin al-Rabi', whom Sufyan al-Thawri and Shu'bah declared thiqah (trustworthy), while a group (of scholars) called him da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1045