Dhikr vs. Takfir

We were sitting in a circle in al-Kufa when a man among us said: "We were sitting with Abu Hurayra whereupon a young man passed by. A man sitting with us said: 'This is a Kafir from among the people of the Fire.' Abu Hurayra rose and went to speak with the young man, asking him: 'Who are you?' He replied: 'Son-and-so, son of So-and-so.' Abu Hurayra said: 'Allah have mercy on your father!' The young man was looking around, so he asked him: 'What are you looking for?' He replied: 'I have not prayed yet.' Abu Hurayra said: 'So you pray?' The young man replied: 'Subhan Allah!' Abu Hurayra: 'And you say Subhan Allah?' He said: 'La Ilaha illAllah!!' Abu Hurayra: 'And you say La Ilaha illAllah?' The young man said: 'I would prefer not to leave Salat even if I were given all there is on the face of the earth.' Abu Hurayra said: 'Allah have mercy on you. Allah have mercy on you. Allah have mercy on you.' Then he came back to his seat in the circle and said: 'I heard the Messenger of Allah say: "Whoever bears testimony against a Muslim of which the latter is not deserving, let him prepare for his seat in the Fire."'"

2. From `Ubayd Allah ibn `Umar, from Nafi`:

A man said to Ibn `Umar: "I have a neighbor who bears witness against me that I commit shirk." He replied: "Say: 'La Ilaha illAllah,' you will make him a liar."

3. From Sawwar ibn Shabib al-A`raji:

I was sitting in Ibn `Umar's house when a man came and said: "O Ibn `Umar! There are groups of people bearing witness against us and attributing to us kufr and shirk." Ibn `Umar replied: "Woe to you! Did you not say: 'La Ilaha IllAllah'?!" Whereupon the entire household began to say La Ilaha IllAllah until the house was shaking.

4. From al-A`mash, from Abu Sufyan:

We came to see Jabir ibn `Abd Allah who lived in Makkah and resided with the Banu Fihr. A man asked him: "Did you [the Companions] use to call anyone from the People of the Qibla [i.e. Muslims], 'Mushrik'?" He replied: "I seek refuge in Allah." The man continued: "Did you call anyone from them 'Kafir'?" He said: "No."