Is starting Sa'i from Safa a prerequisite?

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Order is a condition for the validity of Sa'i; one must commence at Safa. If one begins at Marwah, that circuit is not counted. Once at Safa, subsequent actions are counted. This is based on the Prophet's action and the statement: 'We begin with that which Allah began with.' This is the position of Al-Hasan, Malik, Al-Shafi'i, Al-Awza'i, and the Companions of opinion (Aṣḥāb al-Ra’y). Ibn Abbas affirmed this based on the Quranic verse {Indeed, Safa and Marwah are among the symbols of Allah} (Quran 2:158), commanding to follow the order mentioned in the Quran.