Hunaydah bin Khalid (may Allah have mercy on him) said that he came to Sayyiduna Ali (may Allah be pleased with him) while he was carrying out a hadd punishment on someone. He (may Allah be pleased with him) said to the executioner: Strike, give every limb its due, and avoid the face and the private parts.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17581