Abd al-Rahman bin Yazid narrated that people said to Sayyiduna Salman (may Allah be pleased with him): "Your Prophet teaches you everything, even the manner of relieving yourselves?" He said: "Yes! The Prophet (peace and blessings be upon him) forbade us to face the qiblah while defecating or urinating, he also forbade using fewer than three stones, and he also forbade cleaning ourselves with dung or bone."
(b) In one narration: "None of you should clean himself with fewer than three stones."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 499