Muhammad bin Abdullah bin Abd al-Hakam says that he heard Imam al-Shafi'i (may Allah have mercy on him) saying: Muhammad bin al-Hasan said to me: "Had I known that Sayf bin Sulayman narrates the hadith of the oath along with one witness, I would have declared it unsound." He said: I said: "O Abu Abdullah, if you had declared it unsound, it would have become unsound."
Sayf bin Sulayman al-Makki is thiqah thabt (trustworthy and firm) according to the imams of transmission.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20638