Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
According to the strongest scholarly opinion, Hajj without performing Sa`i between As-Safa and Al-Marwah is invalid, as Sa`i is a pillar of Hajj. If missed, one must return to complete it or follow the ruling of a Muhsar if unable to return.
Answering your question, the Fatwa Center at IslamWeb, states:
Is Sa`i a Pillar or Obligation in Hajj?
According to the sound (preponderant) opinion, Sa`i between As-Safa and Al-Marwah is a pillar of both Hajj and `Umrah, without which these rites are not valid.
What Happens If Sa‘i Is Missed During Hajj?
Therefore, if you performed Hajj—whether Tamattu`, Qiran, or Ifrad—and did not perform Sa`i after Tawaf al-Qudum (the arrival circumambulation), then your state of ihram has not yet ended with the second (major) stage of release. This is because the second stage of release only occurs after completing both Tawaf al-Ifadh and Sa`i.
This ruling applies whether you are performing Hajj for yourself or on behalf of someone else.
Accordingly, you must return to Makkah and complete the missed rounds of Sa`i. As a precaution, you should repeat both the Tawaf and the Sa`i from the beginning, in order to avoid scholarly disagreement—since some scholars require that Tawaf and Sa`I be performed consecutively, and that the rounds of Sa`i be continuous as well.
Once you complete that, your ihram will be properly concluded.
What If You Can’t Return to Makkah to Complete Sa‘i?
However, if you are unable to return to Makkah, you will fall under the ruling of the Muhsar (one who is prevented from completing Hajj due to a valid excuse). In this case, you must exit your ihram by slaughtering a hadi (sacrificial animal). If you are also unable to offer the hadi, then you must fast ten days, after which your ihram will be considered complete.
This represents the first opinion regarding Sa`i.
The second opinion holds that Sa`i is obligatory but not a pillar, and that omitting it does not invalidate Hajj, though it does require a compensatory sacrifice.
However, the first opinion is the stronger and more correct view according to our scholars.
Almighty Allah knows best.
Source: Islamweb.net with editorial changes.