What is the ruling on someone who entered the Grand Mosque in Makkah and prayed two rak'ahs of greeting out of ignorance, without performing tawaf? And is he sinful?

There is no sin upon one who enters the Sacred Mosque (Masjid al-Haram) and begins with the salutation of the mosque (tahiyyat al-masjid) instead of circumambulating (tawaf); this is because beginning with tawaf is a Sunnah, not an obligation. Imam al-Nawawi holds the view that beginning with tawaf is recommended for everyone who enters, unless there is a fear of missing an obligatory prayer, a regular Sunnah prayer, or a congregational prayer, or if one has a missed obligatory prayer to perform; in such cases, these take precedence over tawaf. Some scholars are of the opinion that beginning with tawaf is recommended for those who intend to perform tawaf. However, for one who enters the mosque for prayer or a similar purpose, they should begin with the salutation of the mosque, which is two rak'ahs of prayer. Sheikh al-Uthaymeen clarified that tawaf is the salutation for the Sacred Mosque for those who intend to perform tawaf; otherwise, two rak'ahs of prayer serve as the salutation, just like in other mosques.