It is narrated from al-Sha'bi (may Allah have mercy on him) that in a hadd (prescribed punishment) the testimony of a man upon the testimony of another man is not valid, nor is a guarantee valid in a hadd.
A hadith has also been narrated on this with a weak chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضمان / 11416