How do you explain the saying of Ibn Abbas: "The pillars of Islam are three: bearing witness that there is no god but Allah, prayer, and fasting Ramadan; whoever abandons one of them has disbelieved," alongside your statement that one who does not fast Ramadan remains a Muslim?

The hadith narrated from Ibn Abbas (may Allah be pleased with them both): "The bonds of Islam and the foundations of the religion are three, upon which Islam is built. Whoever abandons one of them is an infidel on account of it, whose blood is lawful: the testimony that there is no god but Allah, the obligatory prayer, and the fasting of Ramadan," is a weak hadith, not sound, whether narrated as marfu' (attributed to the Prophet) or mawquf (attributed to a Companion). This is due to defects in its chain of narration, including: the weakness of Mu'ammal ibn Isma'il, the disagreement among narrators regarding its attribution to the Prophet or its cessation at Ibn Abbas, and the weakness of 'Amr ibn Malik al-Nakari. Even if its authenticity were assumed, it would be interpreted as a severe warning or as referring to one who deems the abandonment permissible.

As for the authentic hadith, it is what Aisha (may Allah be pleased with her) narrated that the Messenger of Allah (peace be upon him) said: "There are three things I swear by: Allah Almighty will not equate one who has a share in Islam with one who has no share in it, and the shares of Islam are three: prayer, fasting, and zakat. And Allah Almighty will not appoint a servant as His guardian in this world then entrust him to another on the Day of Resurrection. And no man loves a people but Allah Almighty will make him with them. And a fourth thing, if I were to swear by it, I would hope not to commit a sin: Allah Almighty does not conceal a servant's faults in this world except that He will conceal them on the Day of Resurrection." This hadith has an authentic chain of narration.