He remembered after he left the mosque that he had forgotten to do one of the circuits of tawaaf al-ifaadah

Praise be to Allah.

If the pilgrim does tawaaf al-ifaadah and forgets one of the circuits, and a long time has passed, then he should repeat the tawaaf, but if only a short time has passed, then he should do the circuit that he forgot. 

And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote. 

Standing Committee for Academic Research and Issuing Fatwas 

Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn Qa’ood. 

Fataawa al-Lajnah al-Daa’imah (11/253)