أَدِلَّةٌ مِنَ السِّيرَةِ عَلَى تَكْفِيرِ مَنْ قَالَ لَا إلَهَ إلاَّ اللهُ
Evidences from Islâmic History for Declaring Takfîr upon the Person Who Says La Ilaha Illallâh[The Wars of Riddah (Apostasy) and the Matter of Declaring Takfîr upon the Person Who Utters La Ilaha Illallâh]When the Arabs obeyed Rasûlullâh Sallallâhu Alayhi wa Sallam, entered the religion of Allâh in crowds, and began fighting against the non-Arabs, Allâhu Taâlâ chose for Rasûlullâh Sallallâhu Alayhi wa Sallam what was with Himself. So, Rasûlullâh Sallallâhu Alayhi wa Sallam passed away after residing in Madinah for ten years. When he passed away, he had preached the message and conveyed the trust. After this, the famous Riddah occurred.
Shaykh'ul Islâm Muhammad bin Abd'il Wahhâb Rahimahullâh1
The reason for this is that when Rasûlullâh Sallallâhu Alayhi wa Sallam died, most of those who embraced Islâm apostated and a grave trial occurred. Allâhu Taâlâ, made steadfast upon the religion those whom He blessed with being steadfast by means of Abû Bakr as-Siddîq Radiyallâhu Anh. For Abû Bakr as-Siddîq Radiyallâhu Anh resisted in a manner which none from the companions adopted. He reminded them what they had forgotten and taught them what they were ignorant of. He encouraged them when they were afraid. Thus, Allâhu Taâlâ made the religion of Islâm firm with him. May Allâhu Taâlâ make us from those who follow him and follow what his companions undertook.
Allâhu Taâlâ said,
"O you who believe! Whoever from among you turns back from his religion, Allâh will bring a people whom He will love and they will love Him; humble towards the believers, stern towards the disbelievers, fighting in the Way of Allâh, [and never fear of the blame of the blamers. That is the grace of Allâh which He bestows on whom He wills. And Allâh is All-Sufficient for His creatures' needs, All-Knower.]2" (al-Mâ'idah 5/54)
Al-Hasan said, "By Allâh, they are Abû Bakr Radiyallâhu Anh and his companions."
1- "Mukhtasar Sîrat'ir Rasûl", Mu'allafât'ush Shaykh, 4/36-50; ad-Durar'us Saniyyah, 9/382-396.
2- The addition in parenthesis is found in ad-Durar'us Saniyyah.