Is It Permissible to Do a Voluntary Sa`i?
Question

Is it permissible to do a voluntary Sa`i?
Praise be to Allah.Shaykh Ibn `Uthaymin (may Allah have mercy on him) said:
“It is not permissible to do a voluntary Sa`i, because Sa`i is only prescribed as part of the rituals of Hajj or `Umrah. Allah says (interpretation of the meaning):
 “Verily, As-Safa and Al-Marwah (two mountains in Makkah) are of the Symbols of Allah. So it is not a sin on him who performs Hajj or `Umrah of the House (the Ka`bah at Makkah) to perform the going (Tawaf) between them (As-Safa and Al-Marwah). And whoever does good voluntarily, then verily, Allah is All-Recogniser, All-Knower.” [Al-Baqarah 2:158]” 
(Fatawa Arkan Al-Islam, p. 540)
For more details, please refer to the following answers: 175579 , 34695 , 109274 , 13369 , 109286 , and 209165 .
And Allah knows best.