Is it permissible to perform Sa'i voluntarily?

It is not permissible to perform the Sa'i (ritual circumambulation between Safa and Marwah) voluntarily, as it is prescribed only as part of the rituals of Nusuk (Hajj and 'Umrah). This is based on the Almighty’s saying: "Indeed, As-Safa and Al-Marwah are among the symbols of Allah. So whoever performs Hajj to the House or performs 'Umrah, there is no blame upon him for circumambulating between them. And whoever volunteers good - then indeed, Allah is appreciative and Knowing." (Quran 2:158)