Praise be to Allah.
Shaykh Ibn ‘Uthaymeen said:
It is not permissible to do a voluntary saa’i, because saa’i is only prescribed as part of the rituals of Hajj and ‘Umrah. Allaah says (interpretation of the meaning):
“Verily, As-Safa and Al-Marwah (two mountains in Makkah) are of the Symbols of Allaah. So it is not a sin on him who performs Hajj or ‘Umrah (pilgrimage) of the House (the Ka‘bah at Makkah) to perform the going (Tawaaf) between them (As-Safa and Al-Marwah). And whoever does good voluntarily, then verily, Allaah is All-Recogniser, All-Knower”