Explanation

Sa'i between Safa and Marwah is an essential rite (wajib)

Running b/w Safa & Marwa seven times is called Sayee. 

Muslims travel back and forth between them 

It is an integral part of the Hajj and Umrah in Islam.

After performing Say’ee Hujjaj go to Mina.

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It is done to symbolize Hajar's (Wive of Prophet Ibraheem, Peace be upon them both) search for water for her son Prophet Ismael (Peace be upon Him).